docs: revamp README and add contributor docs

- README: quick-links bar, compositor-support table, module list grouped by
  theme with links to the wiki, getting-started section, collapsible
  dependency blocks, updated badges (CI, release), and a note that module
  docs live in man/ (auto-synced to the wiki)
- add CONTRIBUTING.md (dev build, code style, docs workflow, PR checklist)
- add issue templates (bug report, feature request) and a PR template

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CONTRIBUTING.md

@@ -0,0 +1,45 @@
+# Contributing to Waybar
+
+Thanks for helping improve Waybar! This guide covers the essentials.
+
+## Building for development
+
+```bash
+meson setup build
+ninja -C build
+./build/waybar          # run your build directly
+```
+
+Enable all optional modules while developing:
+
+```bash
+meson setup build -Dexperimental=true
+```
+
+## Code style
+
+Waybar follows [Google's C++ style guide](https://google.github.io/styleguide/cppguide.html).
+Format your changes before committing:
+
+```bash
+clang-format -i <files>
+```
+
+CI runs `clang-format` and a full build on Linux and FreeBSD — please make sure
+both pass.
+
+## Documentation
+
+Module documentation lives in [`man/`](man) as scdoc man pages, **not** in the
+wiki. Editing a man page and merging to `master` regenerates the matching wiki
+page automatically (see [`.github/wiki`](.github/wiki)). When you add a module,
+add its man page and a line in [`.github/wiki/mapping.json`](.github/wiki/mapping.json).
+
+## Pull requests
+
+- Branch from `master` and keep each PR focused on one change.
+- Describe what changed and why; link any related issues.
+- Add or update the man page for every user-facing option you introduce.
+- Build and test against the module(s) you touched.
+
+Have fun :)