Better release notes generation and automated release process

[Pierre-Sassoulas]

Current problem

• [x] We have to cherry-pick manually #7776
• [x] contributor-txt pollutes the contributor file with a header because it does not detect the existing header fixed in #7656
• [x] The contributor needs to be sorted manually because contributor-txt does not handle it
• [ ] The release notes are generated as rst but github want markdown
• [ ] The new release note file is not included in any toctree
• [ ] Only the first link is generated when there is two or more issues referenced in a fragment
• [x] if false-positive is used instead of false_positive the changelog check is green but the result is not automatically added to the release note see #7369 #7903
• [x] The release note generated from patch create a duplicated title #7393 fixed in #7589
• [x] The release date is not automatically added in the documentation (0fba80c8c)
• [ ] Consider automating the deduplication of changelog fragments during patch releases (ref #9301)

Desired solution

Markdown creation so it can be used in both context, other issue automatically fixed

Additional context

Noticed during the release of 2.15.0.