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.
