Support rendering changelog as MarkDown files

[felixfontein]

This is a project to render changelogs as MarkDown files next to ReStructuredText files (the current default).

This is necessary since GitHub screwed up their RST preview, which results in many collection changelogs to no longer render. This also affects the main Ansible changelogs.

Ref: https://github.com/orgs/community/discussions/86715

[felixfontein]

The latest commits allow to write the changelog in multiple formats (RST and MD are supported right now). The MD result looks like this: https://github.com/felixfontein/antsibull-changelog/blob/markdown/CHANGELOG.md (compare this to https://github.com/felixfontein/antsibull-changelog/blob/markdown/CHANGELOG.rst).

[felixfontein]

This is ready for review!

[felixfontein]

@oraNod @gotmax23 I plan to merge this tomorrow and create a new antsibull-changelog release to get this out (and convert the primary changelog of various collections I maintain to MD).