misc/genreadme,tempfork/pkgdoc,tsnet: generate README.md files from godoc

Adds a CI check to keep opted-in directories' README.md files in sync
with their package godoc. For now tsnet (and its sub-packages under
tsnet/example) is the only opted-in tree. The list of directories
lives in misc/genreadme/genreadme.go as defaultRoots, so CI and humans
both just run `./tool/go run ./misc/genreadme` with no arguments.

The check piggybacks on the existing go_generate job in test.yml and
fails if any README.md is out of date, pointing the user at the same
command.

Along the way:

 - tempfork/pkgdoc now emits Markdown instead of plain text: headings
   become level-2 with no {#hdr-...} anchors, and [Symbol] doc links
   resolve to pkg.go.dev URLs, including for symbols in the current
   package (which the default Printer would otherwise emit as bare
   #Name fragments with no backing anchor in a README). Parsing no
   longer uses parser.ImportsOnly, so doc.Package knows the package's
   symbols and can resolve [Symbol] links at all.

 - genreadme also emits a pkg.go.dev Go Reference badge at the top of
   a library package's README; suppressed for package main.

 - tsnet/tsnet.go's package godoc is expanded in idiomatic godoc
   syntax — [Type], [Type.Method], reference-style [link]: URL
   definitions — rather than Markdown-flavored [text](url) or
   backtick-quoted identifiers, so that both pkg.go.dev and the
   generated README.md render cleanly from a single source.

Fixes #19431
Fixes #19483
Fixes #19470

Change-Id: I8ca37e9e7b3bd446b8bfa7a91ac548f142688cb1
Co-authored-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Signed-off-by: Walter Poupore <walterp@tailscale.com>
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

.github/workflows/test.yml

@@ -787,6 +787,14 @@ jobs:
         echo
         echo
         git diff --name-only --exit-code || (echo "The files above need updating. Please run 'go generate'."; exit 1)
+    - name: check that 'genreadme' is clean
+      working-directory: src
+      run: |
+        ./tool/go run ./misc/genreadme
+        git add -N . # ensure untracked files are noticed
+        echo
+        echo
+        git diff --name-only --exit-code || (echo "The files above need updating. Please run './tool/go run ./misc/genreadme'."; exit 1)
 
   make_tidy:
     runs-on: ubuntu-24.04