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/ISSUE_TEMPLATE/feature_request.md

@@ -0,0 +1,19 @@
+---
+name: Feature request
+about: Suggest an idea or improvement for Waybar
+title: ''
+labels: enhancement
+assignees: ''
+---
+
+**Is your feature request related to a problem?**
+A clear and concise description of the problem, e.g. "I'm frustrated when […]".
+
+**Describe the solution you'd like**
+What you want to happen.
+
+**Describe alternatives you've considered**
+Any alternative solutions or features you've thought about.
+
+**Additional context**
+Which module does this concern? Add any other context or mockups here.