Bring the README up to date

[excid3]

Documentation only; build fingerprints are unchanged (the series.yml edit is a comment).

• Intro: what the tarballs are, what's bundled, which distributions they run on
• Usage: mise setting, plain tarball, asdf; plain-version vs build-revision releases
• Table of series, versions, bundled OpenSSL and YJIT
• Rails applications: which Rails each series was deployed with (3.2 on 1.8.7 through 8.1 on 4.0) and what the deploy covered
• Native gems: bundled headers and static libraries, rbconfig rewritten for the new location, internal functions not linkable from the static libruby, system terminfo (Keep build-tree details out of native gems #16)
• Alongside the system's OpenSSL: what Hide the bundled OpenSSL's symbols from other libraries #12 and Stop old openssl extensions clashing with OpenSSL 3, add missing 2.7/3.0/3.1/3.2.0 #14 do and how it is tested
• End-of-life Rubies: no longer described as local-development-only; both targets are released; caveats (1.8 wrapper and net/http CA store, preinstalled Bundler/RubyGems, absolute shebangs from old RubyGems, MJIT compiler, Puma on 1.9.3)
• Build checks, and that PR CI only builds 3.4.1
• Releases: autobump, rebuilding after a packaging change (only / replace_all)
• Removed RESEND_API_KEY/NOTIFY_EMAIL (no workflow uses them); corrected the 3.2 baseruby note; credit jdx/ruby
• Sections for people using the builds now come before the ones for building them