feat: define myst_substitutions per version

[dgarcia360]

Allow defining myst_substitutions per branch or version instead of falling back to the default branch values.

This follows a similar approach to the rst_prolog variable, but applied to Myst.

How to test

Tested locally, though it’s tricky to reproduce. The steps are:

1. Replace the PyPI extension with the local extension.
2. Define myst_substitutions variable with different values in multiple branches.
3. Run multiversion.

Next steps

After merging, I will release a new version of the extension.

[annastuchlik]

Let's merge it.