Convert to MySTMD & enable PR previews on RTD

[lundybernard]

• Convert file format to MyST, built on mystmd (see https://github.com/statistical-python/statistical-python.org)
• Build using GH Actions
• Add PR previews via RTD

[stefanv]

@lundybernard As per the community call this morning, let's port the root site to mystmd and try previews using RTD, still using GH Pages for deployment. Thank you!

[stefanv]

I've updated the PR description accordingly.