[BLOCKED ON UPSTREAM] Substitutions from MyST frontmatter does not work

[Bizordec]

MyST allow to define substitutions in frontmatter, but those are not visible in sphinx-substitution-extensions.
See: https://myst-parser.readthedocs.io/en/latest/syntax/optional.html#substitutions-with-jinja2

Related myst-parser PR that might help: executablebooks/MyST-Parser#966

[adamtheturtle]

This should now be done. See the latest release and please create a new issue if it isn't working for you.