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/bug_report.md

@@ -0,0 +1,37 @@
+---
+name: Bug report
+about: Report a problem with Waybar
+title: ''
+labels: bug
+assignees: ''
+---
+
+**Describe the bug**
+A clear and concise description of what the bug is.
+
+**To reproduce**
+Steps to reproduce the behaviour, including a minimal config that triggers it:
+
+```jsonc
+// minimal config
+```
+
+**Expected behaviour**
+What you expected to happen.
+
+**Screenshots**
+If applicable, add screenshots to help explain the problem.
+
+**Environment**
+- Waybar version (`waybar --version`):
+- Installed from (distro package / source / other):
+- Compositor and version (Sway, Hyprland, River, …):
+
+**Debug output**
+Run `waybar -l debug` and paste any relevant output:
+
+```
+```
+
+**Additional context**
+Anything else that might help.