docs: consolidate Community sections in README (#2736)

* docs: consolidate Community sections in README

Replace four separate Community sections (Extensions, Presets,
Walkthroughs, Friends) with a single consolidated section containing
a bullet list, one shared disclaimer, and both publishing guide links.

* fix: broken community anchor links and missing Hermes hook note injection

- Update README.md and extensions/README.md to point community
  extension links to the docs site instead of removed section anchor
- Add post_process_skill_content() call in Hermes setup() so hook
  command notes are injected into generated skills
- Add Hermes test override for test_hook_sections_explain_dotted_command_conversion
  with Path.home() monkeypatch