[mystmd] Setup branch to test migration to mystmd

[mmcky]

@kp992 -- we should setup a branch on this repository to migrate the build system to mystmd

[kp992]

Sure @mmcky

[kp992]

I have started a branch myst_build to perform the migration. Is there a way we can test the deployment?

[mmcky]

Closing as complete: the migration branch this issue asked for exists and is actively maintained as PR #363. Ongoing migration tracking is consolidated in #593.