Enable myst builds to be more reproducible #2586
Description
Proposal
When myst build runs, it pulls down various assets from the internet if they're not cached locally. Documentation mentions that the latest version of the template will be downloaded. Based upon build output in jupyter book docs referencing /refs/heads/main.zip, it sounds like themes also pull from the latest version. (if I knew the relationship between templates and themes, I've forgotten)
As a result, if someone builds a myst project several months apart and doesn't change anything, even pinning mystmd, they still might get different results based on what has changed upstream.
It'd be nice if there was an easy way for users to be aware of all of the upstream run-time dependencies, and also pin their versions somehow.
Additional notes
Perhaps one way is to fork the upstream theme and reference that in the project? Is there an easier way? (i.e. for users who develop their own myst sites but know less about the whole architecture of myst)
In this particular issue I'm focused on the myst machinery and less so on the execution environment. (#1265) Perhaps they can be addressed in the same way though.