chore(docs): retire wiki, chunked HTML, and split-manual pipeline

docs/manual/** plus the Astro Starlight site (https://fish-config-docs.pages.dev/)
replace the Gitea wiki (docs/wiki/) and chunked offline HTML (docs/html/,
docs/html-style.html) as the published documentation surface. Retires the
now-unused docs/split-wiki.py and docs/split-manual.py generators alongside them.

- config-help --html now opens the published site root instead of resolving
  a local sitemap anchor; when a keyword was given it prints a note that deep
  links aren't available yet and to use the site's search box.
- open-url and docs/manual/05-functions/14-miscellaneous.md examples
  repointed from docs/html/index.html to the site URL.
- docs/manual/11-viewing-this-manual.md rewritten to document the four
  access paths: the website, the man page, in-terminal pager, and reading
  docs/manual/** directly. fish-config.index updated to match the new
  headings.
- README.md documentation section and Zoxide attribution link repointed at
  the site; corrected the stale "single source file" claim about
  fish-config.md.
- Regenerated docs/fish-config.md via build-manual.py --concat.

docs/fish-config.index

@@ -291,10 +291,10 @@ local-config=## local.fish
 # ── Section 11: Viewing This Manual ──────────────────────────
 viewing=# 11. VIEWING THIS MANUAL
 manual=# 11. VIEWING THIS MANUAL
-ov=## With ov (recommended)
+ov=## In the terminal
 man-page=## As a man page
 manpage=## As a man page
-jump=## Jumping to a section
-html=## In the browser (HTML)
-browser=## In the browser (HTML)
-wiki=## As a wiki
+jump=## In the terminal
+html=## The documentation website
+browser=## The documentation website
+site=## The documentation website