Help: Consolidate 3.8 release notes

Move all development release notes into a new version-specific document:

    tail -q -n +3 Help/release/dev/* > Help/release/3.8.rst
    git rm -- Help/release/dev/*

except the sample topic:

    git checkout HEAD -- Help/release/dev/0-sample-topic.rst

Reference the new document from the release notes index document.
Add a title and intro sentence to the new document by hand.