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/model-provider-plugin.md

@@ -257,7 +257,7 @@ acme-inference = "acme_hermes_plugin:register"
 
 …where `acme_hermes_plugin:register` is a function that calls `register_provider(profile)`. The general PluginManager picks up entry-point plugins during `discover_and_load()`. For `kind: model-provider` pip plugins, you still need to declare the kind in your manifest (or rely on the source-text heuristic).
 
-See [Building a Hermes Plugin](/guides/build-a-hermes-plugin#distribute-via-pip) for the full entry-points setup.
+See [Building a Hermes Plugin](/developer-guide/plugins#distribute-via-pip) for the full entry-points setup.
 
 ## Related pages
 
@@ -265,4 +265,4 @@ See [Building a Hermes Plugin](/guides/build-a-hermes-plugin#distribute-via-pip)
 - [Adding Providers](/developer-guide/adding-providers) — end-to-end checklist for new inference backends (covers both the fast plugin path and the full CLI/auth integration)
 - [Memory Provider Plugins](/developer-guide/memory-provider-plugin)
 - [Context Engine Plugins](/developer-guide/context-engine-plugin)
-- [Building a Hermes Plugin](/guides/build-a-hermes-plugin) — general plugin authoring
+- [Building a Hermes Plugin](/developer-guide/plugins) — general plugin authoring