docs: Plugins subcategory under Extending + secret-source plugin guide + 1Password sidebar fix (#59613)

* docs(secrets): secret-source plugin developer guide + sidebar registration for 1Password page

- New developer-guide/secret-source-plugin.md: SecretSource contract
  (never raises/prompts, fetch-only, timeout budget), framework-vs-plugin
  ownership table, mapped-vs-bulk shape guidance, run_secret_cli()
  subprocess-safety, registration + timing note, conformance kit usage,
  ErrorKind reference.
- Register user-guide/secrets/onepassword in the sidebar (page shipped
  in #59498 but was not listed, so it was unreachable from nav).
- Cross-link the user-guide plugin section to the new dev guide.

* docs: group all plugin guides under a Plugins subcategory in Extending

- Move guides/build-a-hermes-plugin.md -> developer-guide/plugins/index.md
  (both locales) and make it the category landing page (slug pinned to
  /developer-guide/plugins).
- New sidebar subcategory Developer Guide > Extending > Plugins holding
  the general guide + all 8 provider-plugin docs (llm-access, memory,
  context-engine, secret-source, model, image-gen, video-gen, web-search);
  provider-doc URLs unchanged.
- Client redirect /guides/build-a-hermes-plugin -> /developer-guide/plugins.
- Update 30 cross-links across both locales.

website/docs/developer-guide/adding-tools.md

@@ -14,7 +14,7 @@ If you want a personal, project-local, or otherwise custom tool without
 modifying Hermes core, use the plugin route instead:
 
 - [Plugins](/user-guide/features/plugins)
-- [Build a Hermes Plugin](/guides/build-a-hermes-plugin)
+- [Build a Hermes Plugin](/developer-guide/plugins)
 
 Default to plugins for most custom tool creation. Only follow this page when
 you explicitly want to ship a new built-in tool in `tools/` and `toolsets.py`.