Readthedocs toctree clean-up

[robrap]

• A recent Issue/PR improved the docs with some new index pages for app-level docs, but they were dropped into the toctree at the top-level:
  • docs: improve app-level docs discoverability (#35014) #38656
• This brought attention to some other toctree issues.

The requirements are human driven, but this plan and PR are AI generated:

• docs: reorganize toctree — merge how-tos/TinyMCE into Concepts, move app docs into References #38692
• https://github.com/openedx/openedx-platform/blob/6a13210c51ca74a7f54aa9d4e9167dc32dde049d/docs/decisions/toctree-reorganization-plan.md

1. The plan needs review to see if others agree with the proposed reorganization details.
2. Someone who can build the docs locally would need to take over authorship of this PR. It has not yet been human reviewed.

[robrap]

@feanil @irfanuddinahmad: Even if you just review the requirements of the plan (not the implementation), you can see if you agree with the proposed reorganization. I don't plan on completing this due to the challenge of getting the build to work locally.