FIX: Sort out the sync submodule issue 🤔

[daquinteroflex]

After many closed branches attempts including with git subtree

[daquinteroflex]

So I think this kind-of sorts it:

This PR seems to be building alright
https://readthedocs.com/projects/flexcompute-tidy3ddocumentation/builds/1938919/

PR docs hosted here:
https://docs.flexcompute.com/projects/tidy3d/en/dario-fix_sync_with_submodule/?next=https%3A%2F%2Fdocs.flexcompute.com%2Fprojects%2Ftidy3d%2Fen%2Fdario-fix_sync_with_submodule%2F&ticket=ST-1704812510-leWqwdS0RktzZH1NOxwYb2daYqfUXhpl&subproject=tidy3d

I think it should have checked out the right submodule:

Submodule 'docs/notebooks' (git@github.com:flexcompute/tidy3d-notebooks.git) registered for path 'docs/notebooks'
  Cloning into '/home/runner/work/tidy3d/tidy3d/docs/notebooks'...
  From https://github.com/flexcompute/tidy3d-notebooks
   * branch            dfc784c8a2775c6dda3b9dd4c0169a85a523c2ad -> FETCH_HEAD
  Submodule path 'docs/notebooks': checked out 'dfc784c8a2775c6dda3b9dd4c0169a85a523c2ad'

I am still verifying that the exact file in the documentation is the right output file.

Why "kind of" sorted? Really because this is a patch. It works fine for a release and our docs build fine. There is a bug on the create branch action which leaves other old files that shouldn't interact with the main docs build but it's important to know they're there. I've raised the issue that it doesn't check out the branch properly with old submodules there GuillaumeFalourd/create-other-repo-branch-action#10 . If we find it valuable I can sit down and fix the action for our intentions.