Add a Jekyll config for the docs site, and switch to Cayman (#3182)

* Add a Jekyll config for the docs site, and switch to Cayman

The site had no _config.yml at all, so it ran on GitHub's defaults: the Primer
theme, and a site title/description quietly sourced from the repo's name and
description fields - meaning an edit in the repo settings UI changed the site's
metadata. This pins both, pins the URL, and picks a theme deliberately.

Also opts into jekyll-sitemap (not part of the always-on GitHub Pages plugin
set), which gives us the sitemap.xml and robots.txt the site was missing, and
excludes docs.csproj, which was being published as site content.

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

* Point the docs index at GitHub Releases for release notes

ReleaseNotes.md is frozen at 3.0 - it says so itself in its first entry - so
the index was sending people to a page whose top line tells them to go
somewhere else. Make Releases the primary link and keep the old page in
parentheses for the historical record.

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

* Point the READMEs at GitHub Releases for release notes

Same fix as the docs index, in the two remaining places that sent people to
the frozen page - including the README embedded in the NuGet package, which is
the one most readers actually see. PackageReleaseNotes already pointed at
Releases, so the package metadata and its README now agree.

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