docs: restructure site navigation — promote features and platforms to top-level (#4116)

Major reorganization of the documentation site for better discoverability
and navigation. 94 pages across 8 top-level sections (was 5).

Structural changes:
- Promote Features from 3-level-deep subcategory to top-level section
  with new Overview hub page categorizing all 26 feature pages
- Promote Messaging Platforms from User Guide subcategory to top-level
  section, add platform comparison matrix (13 platforms x 7 features)
- Create new Integrations section with hub page, grouping MCP, ACP,
  API Server, Honcho, Provider Routing, Fallback Providers
- Extract AI provider content (626 lines) from configuration.md into
  dedicated integrations/providers.md — configuration.md drops from
  1803 to 1178 lines
- Subcategorize Developer Guide into Architecture, Extending, Internals
- Rename "User Guide" to "Using Hermes" for top-level items

Orphan fixes (7 pages now reachable via sidebar):
- build-a-hermes-plugin.md added to Guides
- sms.md added to Messaging Platforms
- context-references.md added to Features > Core
- plugins.md added to Features > Core
- git-worktrees.md added to Using Hermes
- checkpoints-and-rollback.md added to Using Hermes
- checkpoints.md (30-line stub) deleted, superseded by
  checkpoints-and-rollback.md (203 lines)

New files:
- integrations/index.md — Integrations hub page
- integrations/providers.md — AI provider setup (extracted)
- user-guide/features/overview.md — Features hub page

Broken link fixes:
- quickstart.md, faq.md: update context-length-detection anchors
- configuration.md: update checkpoints link
- overview.md: fix checkpoint link path

Docusaurus build verified clean (zero broken links/anchors).

website/docs/user-guide/features/plugins.md

@@ -1,5 +1,8 @@
 ---
-sidebar_position: 20
+sidebar_position: 11
+sidebar_label: "Plugins"
+title: "Plugins"
+description: "Extend Hermes with custom tools, hooks, and integrations via the plugin system"
 ---
 
 # Plugins