Document the various integration points for platform extensions

Description

As far as I know, we're missing a comprehensive guide to all the different ways that plugins or applications can integrate with Open edX. Here's my attempt to fix that

Now that we have [Automatic notifications of ADR changes](https://discuss.openedx.org/t/automatic-notifications-of-adr-changes/1662), my thought is that we can use the ADR watcher bot to watch this document as well, so that anyone who has asked to be notified of architectural decisions will get pinged when someone opens a PR that proposes a new extension point (provided their PR includes a change to this doc file).

Open question: is https://github.com/edx/edx-platform/blob/master/docs/guides/index.rst published somewhere in compiled form? It contains Sphinx directives that GitHub doesn't render, but I couldn't find a compiled version of it. If we're just using GitHub to view it (which I think is great), we should probably drop the `toctree` etc.

Assignee

Nimisha Asthagiri

Reporter

Open Source Pull Request Bot

Labels

Contributor Name

Braden MacDonald

Repo

edx/edx-platform

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