Harden the release pipeline and document the generated surface

[sbokatuk]

• README: fix the stale 3.14.0.1 install pins, and fail CI when README versions drift from Directory.Build.props (build/CheckReadmeVersions.sh)
• Pack the curated docs/release-notes/.md into PackageReleaseNotes, with the releases page as fallback, so nuget.org stops showing empty notes
• Document why DDLogger's error/attributes parameters carry no [NullAllowed]: upstream declares them _Nonnull and the Swift implementation takes a non-optional [String: Any], so letting null through would trade today's managed ArgumentNullException for a native crash. The README's troubleshooting section points at the null-tolerant Log(...) instead.
• Add doc summaries to the 26 main entry types in the ApiDefinitions files; the binding generator carries them into the packages' XML docs (measured, not assumed), so DDDatadog, DDConfiguration, the per-module Enable types, DDRUMMonitor, DDLogger and the header writers now have IntelliSense
• Turn the trim and AOT analyzers on permanently; the whole surface analyses clean today, and IsTrimmable stays deliberately unset - the props explain
• Add build/DiffSwiftHeaders.sh and docs/regenerating-bindings.md: the header diff is the upgrade work list while Objective Sharpie cannot regenerate, and the script also yields the SHA-256 checksums.txt records
• Document the net8 sunset: dropped in the first release after .NET 8 leaves support on 10 November 2026