docs!: Convert Open edX learner's audience to edx.org

[sarina]

• docs!: Convert Open edX learner's audience to edx.org
• build: Change name of open_edx_students dir to edx_students

We've moved the Open edX learner's guide to https://docs.openedx.org/en/latest/learners/index.html
so update the current learner's guide to indicate it is specific to edx.org as we've found
many references in that guide that don't apply to Open edX courses.

We don't build the learner's guide as a check but here it is locally:

[sarina]

@jristau1984 - I was planning to remove the Open edX Learner's guide (see #2323 ) since we migrated the open learning guide to https://docs.openedx.org/en/latest/learners/index.html but @feanil let me know you didn't want us to remove these docs. What do you think about this change, which renames the directory for clarity, and adds a banner to the top of https://edx.readthedocs.io/projects/open-edx-learner-guide/en/latest/ (as well as changes the title and icon to edX-specific).

Longer term this should be on your documentation list to combine this learner guide with the pages in en_us/edx_students to create one learner's guide for edx.org.

Lastly I'll note this guide isn't linked to on https://docs.edx.org/ so the only way I think people would get to it is perhaps links in the UI or Googling.

[jristau1984]

This seems okay to me, thanks.

[sarina]

Thanks @jristau1984 !

@feanil I renamed the directory here for clarity, will I need to adjust the build? I'm realizing I'm not sure because the learner's guide is not being built on PRs.

[feanil]

Yea, you'll have to update the build here: https://app.readthedocs.org/dashboard/open-edx-learner-guide/edit/