Docs site: title duplication, stray underlines, favicon (#3183)

* Stop Cayman printing every page title twice

The theme puts the page title in its header banner, and titles_from_headings
derives that title from each page's first heading - which is still sitting in
the body, so every page reads its own title twice in a row. Worst on the rules
pages, where the title is a full sentence. strip_title drops the heading from
the body once it has been promoted; the banner is unaffected.

Claude-Session: https://claude.ai/code/session_01NxkjXSR7CSGzyw89tJJVtw

* Remove setext underlines left behind in Configuration.md

The headings here were converted to ATX (#) at some point without removing the
old underlines, so ten of them are still in the file. Two render as visible
junk - a literal "===" under the page title and a literal "-" under Basic
Configuration Strings - and the other eight become horizontal rules directly
beneath the heading they used to underline.

Claude-Session: https://claude.ai/code/session_01NxkjXSR7CSGzyw89tJJVtw

* Give the docs site a favicon

Uses the existing project logo. The theme's head includes head-custom.html and
ships a version with the favicon line commented out, so overriding that file is
the hook. Serves the SVG to browsers that take one and a 16/32/48 .ico
otherwise - the latter at the site root, so the automatic /favicon.ico probe
finds it too.

Jekyll can only see files under docs/, so the SVG is a copy of the one in
assets/ rather than a reference to it.

Claude-Session: https://claude.ai/code/session_01NxkjXSR7CSGzyw89tJJVtw