Core Data: Add docgen auto-documentation for actions, selectors

[aduth]

This pull request seeks to update the docgen auto-documentation script to generate documentation for actions and selectors of the @wordpress/core-data module.

Implementation notes:

I could see this being iterated upon toward:

• Eliminating the separate docs/tool script in favor of consolidating all data documentation here
  • Mentioned in "Implementation notes" of Documentation: Update docs tool to omit private, experimental, unstable APIs #15188
• Creating a custom formatter for actions / selectors to make them more accurate / relevant
  • Omit actions return
  • Omit selectors state argument
• Provide examples for using various core-data selectors
  • See Documentation: Document usage of core entities (getEntityRecords, etc) #12120

Testing Instructions:

Verify there are no local changes after running npm run docs:build in this branch.

[oandregal]

This is beautiful!