Fix or remove broken links in release-notes.md

In the past, release notes files were created per-release, but those
have all been deleted - the earliest such release was 2.5, old enough
that I didn't feel it was necessary to dig through history and find
those notes and restore them, but just deleted the broken links.

Old javadoc links apparently used `.` instead of `/` delimiters for
packages, and left off `.html` suffixes. In theory our server will
automatically deal with the missing suffixes, but won't rewrite paths.

Going back to 1.x days, some documentation was written by adding inner
classes to a DeveloperGuide class, and adding notes there. I've found
the present-day equivelents of each of those old documents, or just
deleted the links where nothing seemed appropriate.