docs(plugins): close the gaps \u2014 image-gen-provider-plugin guide + publishing a skill tap (#20800)

Two pluggable surfaces were mentioned in the interfaces map without a
real authoring guide behind them:

1. **Image-gen backends** — only had 'See bundled examples' pointers.
   Now a full developer-guide/image-gen-provider-plugin.md (270 lines)
   mirroring the memory/context/model provider docs:
   - How discovery works, directory structure, plugin.yaml
   - ImageGenProvider ABC with every overridable method
     (name, display_name, is_available, list_models, default_model,
     get_setup_schema, generate)
   - Full authoring walkthrough with a working MyBackendImageGenProvider
   - Response-format reference (success_response / error_response)
   - Handling b64 vs URL output (save_b64_image helper)
   - User overrides at ~/.hermes/plugins/image_gen/<name>/
   - Testing recipe + pip distribution
   - Reference examples (openai, openai-codex, xai)

2. **Skill taps** — features/skills.md mentioned the CLI commands but
   never explained the repo contract for publishing a tap. Added
   'Publishing a custom skill tap' section under Skills Hub covering:
   - Repo layout (skills/<name>/SKILL.md by default)
   - Minimal working example
   - Non-default path configuration (taps.json)
   - Installing individual skills without subscribing
   - Trust-level handling
   - Full tap management CLI + in-session /skills tap commands

Wired into:
- website/sidebars.ts: image-gen-provider-plugin added to Extending group
- website/docs/user-guide/features/plugins.md: pluggable interfaces
  table + 'What plugins can do' table now link to the real guides
  instead of 'See bundled examples'
- website/docs/guides/build-a-hermes-plugin.md: top info map and
  inline sub-sections updated, 'Full guide:' line added to
  image-gen block, tap section mentions publishing

Verified: docusaurus build SUCCESS, new page renders at
/docs/developer-guide/image-gen-provider-plugin, anchor
#publishing-a-custom-skill-tap resolves from plugins.md +
build-a-hermes-plugin.md. Pre-existing zh-Hans broken links unchanged.

website/sidebars.ts

@@ -211,6 +211,7 @@ const sidebars: SidebarsConfig = {
             'developer-guide/memory-provider-plugin',
             'developer-guide/context-engine-plugin',
             'developer-guide/model-provider-plugin',
+            'developer-guide/image-gen-provider-plugin',
             'developer-guide/creating-skills',
             'developer-guide/extending-the-cli',
           ],