Add deprecations section to ReST documentation

[maxrjones]

Description of proposed changes

This PR adds a deprecations section to the ReST docs for modules that will have deprecations in 6.3

Fixes #5615

Reminders

• Make sure that your code follows our style. Use the other functions/files as a basis.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Describe changes to function behavior and arguments in a comment below the function declaration.
• If adding new functionality, add a detailed description to the documentation and/or an example.

[PaulWessel]

Great!