Improve edx-platform docs and discoverability

[robrap]

This is a follow-up to https://github.com/openedx/edx-platform/issues/33327, which made app-level docs part of the edx-platform readthedocs output.

Improvement ideas:

• Add app-level READMEs
• Add top-level index page to make discovery of app-level docs easier.
• Eventually update OEP-19 with best practices for exposing these for a repo based on experience with edx-platform.

[farhan]

OEO-19 doc