feat(docs): add auto-generated Markdown wiki

Adds docs/split-wiki.py, a Python script that splits fish-config.md
into a multi-page Markdown wiki under docs/wiki/:

- index.md: DESCRIPTION intro + full section ToC
- one file per numbered section (1-configuration-variables.md … 9-viewing-this-manual.md)
- each section page has a nav bar at the top linking to every other section

The build-docs.yml workflow now runs the script alongside pandoc so all
three formats (man page, HTML, wiki) are regenerated together from a
single source in one CI run.

README slimmed down to a pointer to the wiki; docs/fish-config.md §9
gains an "As a wiki" subsection.

docs/fish-config.md

@@ -1314,6 +1314,15 @@ find the real browser binary, then falls back through known browser
 binaries (firefox, chromium, vivaldi, etc.), and finally xdg-open as a
 last resort. Set $fish_help_browser or $BROWSER to override.
 
+## As a wiki
+
+The generated Markdown wiki lives in docs/wiki/. index.md provides the
+project overview and a full table of contents. Each section page has a
+navigation bar at the top linking to every other section.
+
+The wiki is auto-generated from this file by the CI pipeline on every
+push to main that changes docs/fish-config.md.
+
 ## Jumping to a section
 
     help config keybindings