Improve the "Working on edx-platform Python dependencies" documentation

[bradenmacdonald]

I couldn't get the edx-platform to persistently use my bind-mounted version of its dependency openedx-learning. I was about to write a giant post on Discourse to ask for help when I happened to notice that the documentation does contain the answer I needed ("What if my edx-platform package is not automatically bind-mounted?"). The problem was, this key section was separated from the related section ("Working on edx-platform Python dependencies") by unrelated topics like how to run the unit tests.

So, this PR:

1. Moves that important section of the documentation up to where it's more relevant, and adds an explicit call out.
2. Also updates tutor to be able to auto-detect openedx-learning without a plugin - as it is becoming a key platform dependency and at least at OpenCraft, many developers need to use a bind-mounted version of this dependency.

I didn't include a changelog entry as this is primarily a documentation-only change.

[DawoudSheraz]

nit: since openedx-learning will be auto-magically mounted once the PR is merged (ignoring the doc changes), we should add a changelog entry for that.

[bradenmacdonald]

Sure @DawoudSheraz. I added a changelog entry.