docs: Update language docs and include links in the sidebar (#13828)

This PR updates the supported language docs and adds them to the sidebar
for better discoverability.

Release Notes:

- N/A

docs/src/languages/css.md

@@ -1,4 +1,6 @@
 # CSS
 
+CSS support is available natively in Zed.
+
 - Tree Sitter: [tree-sitter-css](https://github.com/tree-sitter/tree-sitter-css)
 - Language Server: N/A