docs: add PR review guides, rework quickstart, slim down installation

Adds two complementary GitHub PR review guides from contest submissions:
- Cron-based PR review agent (from PR #5836 by @dieutx) — polls on a
  schedule, no server needed, teaches skills + memory authoring
- Webhook-based PR review (from PR #6503 by @gaijinkush) — real-time via
  GitHub webhooks, documents previously undocumented webhook feature
Both guides are cross-linked so users can pick the approach that fits.

Reworks quickstart.md by integrating the best content from PR #5744
by @aidil2105:
- Opinionated decision table ('The fastest path')
- Common failure modes table with causes and fixes
- Recovery toolkit sequence
- Session lifecycle verification step
- Better first-chat guidance with example prompts

Slims down installation.md:
- Removes 10-step manual/dev install section (already covered in
  developer-guide/contributing.md)
- Links to Contributing guide for dev setup
- Keeps focused on the automated installer + prerequisites + troubleshooting

website/sidebars.ts

@@ -162,6 +162,8 @@ const sidebars: SidebarsConfig = {
         'guides/cron-troubleshooting',
         'guides/work-with-skills',
         'guides/delegation-patterns',
+        'guides/github-pr-review-agent',
+        'guides/webhook-github-pr-review',
         'guides/migrate-from-openclaw',
         'guides/aws-bedrock',
       ],