Create rtd config file, split requirements, and pin versions

[BrianJKoopman]

Description

This does exactly what we did to fix the rtd builds over in [1].

[1] - simonsobs/socs#209

Motivation and Context

Fixes documentation builds.

How Has This Been Tested?

RTD check will occur on this PR.

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.