[DOCS] Adds the 7.15.2 release notes (elastic#117542)

* [DOCS] Adds the 7.15.2 release notes

* [DOCS] Move ML PRs to APM section

* [DOCS] More ML edits

* Update docs/CHANGELOG.asciidoc

* Adding osquery prs

* Fixed typo.

Co-authored-by: lcawl <lcawley@elastic.co>
Co-authored-by: Brandon Morelli <bmorelli25@gmail.com>
Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>
Co-authored-by: nastasha.solomon <nastasha.solomon@elastic.co>