Showcase usage of .MD format

[Adi Roiban]

From #558 (comment)

We can try to have both a NEWS.rst and NEWS.md file, at the same time.

The NEWS.md can help with the content for the relese
We can also use build --draft --config towncrier-md.toml to generate MD format.

The idea is to do some dog food with Markdown format.

It can be tricky, as we should write the content for the news fragment in a format that is valid in both RST and MD... but I think that this can be done, as the content is not complex.

This can be implemented by having a towncrier-md.toml that will generate NEWS.md

we can run it as part of the release, before RST via build --keep --config towncrier-md.toml