feat(docs): add GitHub mirror icon, README-sourced doc sections, auto-generated TOC

Adds a GitHub social icon to the docs site header alongside the existing
Gitea one, and documents in the README's Contributing section that
git.rootiest.dev is the base repo while the GitHub copy is a one-way
mirror, so forks/issues/PRs should go through Gitea.

Adds Testing, Contributing, Attribution, and License sections to the
manual/man page/site, sourced directly from README.md via a new
`<!-- README: Heading -->` placeholder mechanism in build-manual.py, so
the README stays the single source of truth for those sections instead
of a hand-maintained copy drifting out of sync.

Also converts docs/manual/00-table-of-contents.md from a hand-typed list
to a generated one (mt.walk()-driven), fixing a numbering drift where
Components Reference was omitted and every section after it was off by
one relative to its own manTitle.

docs/manual/00-table-of-contents.md

@@ -7,51 +7,6 @@ sidebar:
   order: 4
 ---
 
-    1.  Configuration Variables
-    2.  PATH Setup
-    3.  Key Bindings
-    4.  Abbreviations
-        4.1  Editors
-        4.2  Navigation and Listing
-        4.3  Git
-        4.4  Terminal Windows, Tabs, and Panes
-        4.5  Chezmoi
-        4.6  Docker
-        4.7  Systemctl
-        4.8  AI Assistants
-        4.9  History Expansion
-        4.10 Miscellaneous
-        4.11 Shell Aliases
-    5.  Functions Reference
-        5.1  File and Directory
-        5.2  Navigation
-        5.3  Editors and Viewers
-        5.4  Git and Version Control
-        5.5  Package Management
-        5.6  Dependency Management
-        5.7  System and Monitoring
-        5.8  Terminal Management
-        5.9  Clipboard
-        5.10 Network
-        5.11 Pager and Logging
-        5.12 AI and Developer Tools
-        5.13 Media and Utilities
-        5.14 Miscellaneous
-    6.  Dependency Catalog
-    7.  Customization
-    8.  Fisher Plugins
-    9.  Installation
-    10. Personalization
-    11. Troubleshooting
-        11.1 Uninstalling and Reverting to Backup
-        11.2 Fish Version Requirement
-        11.3 Enable or Disable Session Logging
-        11.4 Change or Disable the Greeting
-        11.5 Secrets and Machine-Local Configuration
-        11.6 Tool Init Does Nothing (Return Sentinel)
-        11.7 Missing Dependencies
-        11.8 Vi Mode Keybindings
-        11.9 What's with the C1-C6 stuff?
-    12. Viewing This Manual
+<!-- GENERATED: toc -->
 
 ---