✨ Add Gitter chat to docs (#1061)

* implemented chat feature using sidecar from starlette docs

* 🔥 Use external Gitter sidecar JS

* ✨ Re-implement Gitter chat after multi-lang

* 🔥 Remove moved file

Co-authored-by: Aakash Nand <aakashnand@nttv6.jp>
Co-authored-by: Sebastián Ramírez <tiangolo@gmail.com>

docs/en/docs/js/chat.js

@@ -0,0 +1,3 @@
+((window.gitter = {}).chat = {}).options = {
+    room: 'tiangolo/fastapi'
+};

docs/en/mkdocs.yml

@@ -145,3 +145,5 @@ extra_javascript:
 - https://unpkg.com/mermaid@8.4.6/dist/mermaid.min.js
 - js/termynal.js
 - js/custom.js
+- js/chat.js
+- https://sidecar.gitter.im/dist/sidecar.v1.js