[Backport 2.10.x] flatten changelog into current release and release history for ease of maintenance

[jodygarnett]

This is a backport some of:

• flatten changelog into current release and release history for ease of maintenance #7887
• how to update mkdoc env (removed instructions for docker mkdocs use) #8089

Checklist

• I have read the contribution guidelines
• Pull request provided for main branch, backports managed with label
• Good housekeeping of code, cleaning up comments, tests, and documentation
• Clean commit history broken into understandable chucks, avoiding big commits with hundreds of files, cautious of reformatting and whitespace changes
• Clean commit messages, longer verbose messages are encouraged
• API Changes are identified in commit messages
• Testing provided for features or enhancements using automatic tests
• User documentation provided for new features or enhancements in manual
• Build documentation provided for development instructions in README.md files
• Library management using pom.xml dependency management. Update build documentation with intended library use and library tutorials or documentation

Thanks to GeoCat Live and my employer GeoCat for the chance to bring you this documentation update

[jodygarnett]

See preview at https://jodygarnett.github.io/core-geonetwork/2.10/

[jodygarnett]

@juanluisrp has reviewed the change to main #8089

I am going to have to take it as approval for this backport as well.