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- checkpoints.md (30-line stub) deleted, superseded by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/integrations/index.md

@@ -0,0 +1,25 @@
+---
+title: "Integrations"
+sidebar_label: "Overview"
+sidebar_position: 0
+---
+
+# Integrations
+
+Hermes Agent connects to external systems for AI inference, tool servers, IDE workflows, programmatic access, and more. These integrations extend what Hermes can do and where it can run.
+
+## Available Integrations
+
+- **[AI Providers](/docs/user-guide/features/provider-routing)** — Set up and configure inference providers. Hermes works with OpenRouter, Anthropic, OpenAI, Google, and any OpenAI-compatible endpoint. Use `hermes model` to configure interactively.
+
+- **[MCP Servers](/docs/user-guide/features/mcp)** — Connect Hermes to external tool servers via Model Context Protocol. Access tools from GitHub, databases, file systems, browser stacks, internal APIs, and more without writing native Hermes tools.
+
+- **[IDE Integration (ACP)](/docs/user-guide/features/acp)** — Use Hermes Agent inside ACP-compatible editors such as VS Code, Zed, and JetBrains. Hermes runs as an ACP server, rendering chat messages, tool activity, file diffs, and terminal commands inside your editor.
+
+- **[API Server](/docs/user-guide/features/api-server)** — Expose Hermes as an OpenAI-compatible HTTP endpoint. Any frontend that speaks the OpenAI format — Open WebUI, LobeChat, LibreChat, NextChat, ChatBox — can connect and use Hermes as a backend with its full toolset.
+
+- **[Honcho Memory](/docs/user-guide/features/honcho)** — AI-native persistent memory for cross-session user modeling and personalization. Honcho adds deep user modeling via dialectic reasoning on top of Hermes's built-in memory system.
+
+- **[Provider Routing](/docs/user-guide/features/provider-routing)** — Fine-grained control over which underlying AI providers handle your OpenRouter requests. Optimize for cost, speed, or quality with sorting, whitelists, blacklists, and explicit priority ordering.
+
+- **[Fallback Providers](/docs/user-guide/features/fallback-providers)** — Automatic failover to backup LLM providers when your primary model encounters errors. Includes primary model fallback and independent auxiliary task fallback for vision, compression, and web extraction.