chore: upgrade docs to middleman 4.6 (was: nokogiri Ruby compat)

[kt3k]

nokogiri 1.16.5 (#2895) requires Ruby >=3.0 for its precompiled native gem, which conflicted with the docs job's Ruby 2.4.10 pin. Rather than patch around that, this upgrades the whole docs toolchain:

• middleman 3.2.2 → ~> 4.6 (Ruby >=2.7). Drops the forced compass/sass/coffee-script/sprockets baggage the old middleman meta-gem pulled in — the docs source is plain .haml + prebuilt JS/CSS, so none of that was ever actually used. middleman-core 4.6 brings its own modern sassc/haml/kramdown.
• Drops middleman-livereload and middleman-sync — neither is activated in config.rb or invoked from any script; they were unused. Removing middleman-sync also removes its fog/fog-xml chain, which is what pulled in nokogiri in the first place — nokogiri isn't part of the dependency tree anymore at all.
• config.rb: dropped the :ugly haml option (removed in Haml 6, core is unaffected otherwise — asset_hash, :source, :css_dir etc. all work unchanged).
• CI: bumps the docs job to Ruby 3.1.

Verified locally (bundle exec middleman build) and in this PR's own CI: output structure, page count, and asset hashing all match the previous build.