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.

.gitea/workflows/build-docs.yml

@@ -35,11 +35,14 @@ jobs:
                  docs/fish-config.md \
                  -o docs/html/
 
+      - name: Generate wiki markdown
+        run: python3 docs/split-wiki.py
+
       - name: Commit generated docs
         run: |
           git config user.name  "Gitea Actions"
           git config user.email "actions@gitea"
-          git add docs/fish-config.1 docs/html/
+          git add docs/fish-config.1 docs/html/ docs/wiki/
           git diff --cached --quiet && echo "No changes to commit" && exit 0
-          git commit -m "chore(docs): regenerate man page and HTML docs"
+          git commit -m "chore(docs): regenerate man page, HTML docs, and wiki"
           git push