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/verify-manual.py

@@ -11,7 +11,7 @@ from pathlib import Path
 import manualtools as mt
 
 # docs/build-manual.py follows this repo's hyphenated CLI-script naming
-# convention (matching split-manual.py, verify-manual.py), which means it
+# convention (matching verify-manual.py), which means it
 # cannot satisfy a plain `import build_manual` on its own — Python's import
 # statement never treats a hyphen as an underscore. Load it explicitly under
 # the name the tests expect and register it in sys.modules; every later