docs: clean up project documentation

- Convert all headings from underline style to ATX style
- Fix heading hierarchy in INSTALLING.md (H3→H2, H4→H3)
- Move Installing section before Usage in README
- Bump install snippet version from 4.5.1 to 4.7.0
- Add feature request guidance to CONTRIBUTING.md
- Add installer package and .cheat discovery to CLAUDE.md
- Remove unused reference links and dead HTML comments
- Trim trailing whitespace

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CONTRIBUTING.md

@@ -1,5 +1,4 @@
-Contributing
-============
+# Contributing
 
 Thank you for your interest in `cheat`.
 
@@ -11,4 +10,8 @@ Bug reports are still welcome. If you've found a bug, please open an issue in
 the [issue tracker][issues]. Before doing so, please search through the
 existing open issues to make sure it hasn't already been reported.
 
+Feature requests may be filed, but are unlikely to be implemented. The project
+is now mature and the maintainer considers its feature set to be essentially
+complete.
+
 [issues]: https://github.com/cheat/cheat/issues