Docs: add Release Notes #233

The OpenIDM documentation has no Release Notes, and the text still describes old versions: "OpenIDM 4.0" appears 51 times and "OpenIDM 4.5" 14 times, while the current release is 7.1.2.

Proposal

Add a release-notes guide to openidm-doc/src/main/asciidoc (what's new per release since the fork, compatibility, supported Java / repositories / containers, upgrade notes, fixed security advisories), and review the version references in the other guides.