docs: Combine CHANGELOG.rst with CHANGELOG.md (snakemake#1228)

* docs: Convert old changelog from rst -> markdown

Using pandoc -t commonmark CHANGELOG.rst > CHANGELOG.md
Then, adjusted the heading levels

* chore: Use myst-parser for markdown files

Show CHANGELOG.md in docs

* chore: Remove CHANGELOG.rst

* docs: correct changelog rst->md conversion artefacts