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>

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,13 @@
+<!-- Thanks for contributing to Waybar! Please fill in the sections below. -->
+
+**What does this PR do?**
+A short description of the change and the motivation behind it.
+
+**Related issues**
+Closes #
+
+**Checklist**
+- [ ] Code is formatted with `clang-format`
+- [ ] Builds locally (`ninja -C build`)
+- [ ] Man page updated for any new/changed user-facing option (`man/`)
+- [ ] Tested against the affected module(s)