docs: use autodoc_mock_imports for spt3g, so3g

[mhasself]

Description

This allows autodoc to run on readthedocs... without the OCS_DOC_BUILD business I put in before. The aggregator API stuff has not been rendering since HK v2 went in.

Motivation and Context

Recent builds on readthedocs have been failing to process the aggregator.py source file, because some spt3g core.G3* classes were being accessed on import. sphinx has a feature to dodge this. For example, the API section is totally empty.

How Has This Been Tested?

Tested that the docs build locally even if so3g or spt3g aren't available.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• Unless I am preparing a release, I have opened this PR onto the develop branch.

[BrianJKoopman]

This is great! Looks much less prone to breakage than the other work around. Would be great to do the same on socs.