Document how to use the User CSS and User JS core extensions (#9424)

Closes #9394

## Summary

The _User CSS_ and _User JS_ sections already existed in the user documentation, but they did not say where to find the settings. This explains that each extension has a gear icon (_Manage_) next to its switch, and gives the steps to use both extensions.

## Changes

- `docs/en/users/05_Configuration.md`: describe the extension list (switch, gear icon, name), add the steps for User CSS, and the same for User JS with a note that it might ask to sign in again.
- `docs/en/users/05_Configuration.md`, `docs/en/admins/11_Themes.md`, `docs/en/contributing.md`: the "CustomCSS extension" links pointed to FreshRSS/Extensions, where it is no longer available. They now point to the built-in User CSS section.
- `docs/en/admins/15_extensions.md`, `lib/core-extensions/README.md`: list the two core extensions with links to their documentation. Also fix the `.lib/core-extensions` path.
- `CREDITS.md`: add my name.

Only the English documentation is changed.

CREDITS.md

@@ -325,6 +325,7 @@ People are sorted by name so please keep this order.
 * [sirideain](https://github.com/sirideain): [contributions](https://github.com/FreshRSS/FreshRSS/pulls?q=is:pr+author:sirideain)
 * [skrlet13](https://github.com/ghost): [contributions](https://github.com/FreshRSS/FreshRSS/pulls?q=is:pr+author:skrlet13)
 * [Sp3r4z](https://github.com/Sp3r4z): [contributions](https://github.com/FreshRSS/FreshRSS/pulls?q=is:pr+author:Sp3r4z)
+* [spa77k](https://github.com/spa77k): [contributions](https://github.com/FreshRSS/FreshRSS/pulls?q=is:pr+author:spa77k)
 * [stag-enterprises](https://github.com/stag-enterprises): [contributions](https://github.com/FreshRSS/FreshRSS/pulls?q=is:pr+author:stag-enterprises), [Web](https://stag.lol)
 * [Steve Jones](https://github.com/squaregoldfish): [contributions](https://github.com/FreshRSS/FreshRSS/pulls?q=is:pr+author:squaregoldfish)
 * [Strubbl](https://github.com/Strubbl): [contributions](https://github.com/FreshRSS/FreshRSS/pulls?q=is:pr+author:Strubbl)

docs/en/admins/11_Themes.md

@@ -2,7 +2,7 @@
 
 **Note: Currently personal themes are not officially supported and may be overwritten when updating. Be sure to keep backups!**
 
-**For small theme changes, the official [CustomCSS extension](https://github.com/FreshRSS/Extensions) is recommended.**
+**For small theme changes, the built-in [User CSS extension](../users/05_Configuration.md#user-css) is recommended.**
 
 Themes should be installed at `FreshRSS/p/themes/my-theme-name`. Docker users can use:
 

docs/en/admins/15_extensions.md

@@ -48,6 +48,9 @@ metadata.json:
 
 ### pre installed extensions (core extensions)
 
-See folder: `.lib/core-extensions`
+See folder: `./lib/core-extensions`
+
+* `UserCSS`: [User CSS](../users/05_Configuration.md#user-css)
+* `UserJS`: [User JS](../users/05_Configuration.md#user-js)
 
 Important: do not install your chosen extensions here!

docs/en/contributing.md

@@ -71,7 +71,7 @@ This checklist is designed to ensure high-quality system themes while minimizing
 	* Icons should be provided as `svg` files.
 		* Ensure clean code without any unnecessary fragments. SVG files should be optimized using `make svgo`.
 		* The default color for icons is `#666666`; employ CSS filters for recoloring.
-* Utilize CSS variables for colors to facilitate easy color adjustments with [CustomCSS extension](https://github.com/FreshRSS/Extensions).
+* Utilize CSS variables for colors to facilitate easy color adjustments with the [User CSS extension](./users/05_Configuration.md#user-css).
 * Implement a dark mode that aligns with the `.darkMode` CSS class.
 * To streamline maintenance, avoid using CSS preprocessors; instead, employ well-structured plain CSS.
 

docs/en/users/05_Configuration.md

@@ -68,7 +68,7 @@ There’s no accounting for tastes, which is why FreshRSS offers 13 official the
 
 To select a theme, simply scroll through the themes and select one that strikes your fancy. After confirmation, the theme will be applied to the interface.
 
-If you can’t find any themes you like, it’s always possible to [create your own](../developers/04_Frontend/02_Design.md) and [install it](../admins/11_Themes.md). For small theme changes, the official [CustomCSS extension](https://github.com/FreshRSS/Extensions) is recommended.
+If you can’t find any themes you like, it’s always possible to [create your own](../developers/04_Frontend/02_Design.md) and [install it](../admins/11_Themes.md). For small theme changes, the built-in [User CSS extension](#user-css) is recommended.
 
 ### Content width
 
@@ -196,11 +196,21 @@ You can change your email address or password here. The authentication token is
 
 Extensions can be managed from this menu. Note that while extensions can be removed from the web interface, they cannot be added from it.
 
-Some extensions have configurations and these can be changed in the manage page, which opens with the button near the name of the extension.
+Each extension that you can manage is listed with a switch to enable or disable it, followed by a gear icon (⚙️ _Manage_) and the name of the extension.
+
+Some extensions have configurations and these can be changed in the manage page, which opens with the gear icon.
 
 ### User CSS
 
-It gives ability to create user-specific CSS rules to apply in addition of the actual theme.
+_User CSS_ is a core extension shipped with FreshRSS. It gives ability to create user-specific CSS rules to apply in addition of the actual theme.
+
+To use it:
+
+1. Enable _User CSS_ in the list of _User extensions_.
+2. Open its manage page with the gear icon.
+3. Write the rules in the _Additional CSS rules_ field and submit.
+
+The rules only apply to the current user, and only while the extension is enabled.
 
 #### Example: Getting rid of Top Menu Items
 
@@ -233,7 +243,9 @@ The Top Menu within the mobile view might look a little bit cluttered, depending
 
 ### User JS
 
-It gives ability to create user-specific JS.
+_User JS_ is a core extension shipped with FreshRSS. It gives ability to create user-specific JS.
+
+It is used the same way as [User CSS](#user-css): enable _User JS_, open its manage page with the gear icon, write the script in the _Additional JS_ field, and submit. FreshRSS might ask you to sign in again before showing this page.
 
 ## Users
 

lib/core-extensions/README.md

@@ -1,5 +1,8 @@
 # FreshRSS core extensions
 
-This directory contains some core extensions shipped with FreshRSS.
+This directory contains some core extensions shipped with FreshRSS:
+
+* `UserCSS`: [User CSS](https://freshrss.github.io/FreshRSS/en/users/05_Configuration.html#user-css)
+* `UserJS`: [User JS](https://freshrss.github.io/FreshRSS/en/users/05_Configuration.html#user-js)
 
 For custom third-party extensions, use the `./FreshRSS/extensions/` directory instead.