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/manual/05-functions/14-miscellaneous.md

@@ -10,7 +10,7 @@ helpKeywords:
 ## config-help
 
     Synopsis:  config-help [SECTION]
-               config-help [SECTION] --html
+               config-help --html
                config-help [SECTION] --man
                config-help -h | --help
 
@@ -20,11 +20,11 @@ helpKeywords:
     keyword (case-insensitive; checks fish-config.index aliases first).
 
     Flags:
-      --html / -w   Open docs/html/index.html in the default browser.
-                    If SECTION is given, opens at the matching anchor.
-                    Detects the browser via xdg-mime x-scheme-handler/https,
-                    then known binaries, then xdg-open as last resort.
-                    Respects $fish_help_browser and $BROWSER.
+      --html / -w   Open the published documentation website
+                    (https://fish-config-docs.pages.dev/) in the default
+                    browser via xdg-open. Deep links to a section aren't
+                    supported; if SECTION is given, a note points you to the
+                    site's search box instead.
       --man  / -m   Open docs/fish-config.1 via man -l directly.
                     If SECTION is given, jumps to the nearest match.
       --help / -h   Print usage and navigation key reference.
@@ -32,7 +32,6 @@ helpKeywords:
     config-help keybindings
     config-help pkg
     config-help --html
-    config-help pkg --html
     config-help --man
     config-help pkg --man
 
@@ -60,7 +59,7 @@ helpKeywords:
       5. xdg-open            (last resort)
 
     open-url https://git.rootiest.dev/rootiest/fish-config
-    open-url "file://$HOME/.config/fish/docs/html/index.html"
+    open-url -v https://fish-config-docs.pages.dev/
 
     Used internally by config-help --html.