Convert as many direct references to docs.oe.org, and :doc:, to :refs:

[sarina]

Replaces #888 as the rebase was wonky

Link cleanup:

Change as many direct references to docs.openedx.org to relative refs as possible

Change all references to other projects to intersphinx mapping

Change as many direct :doc: directives to :ref:s as possible

Leftover links

The following can't be changed:

• in index.html, you can't put a ref or doc directive within a toctree, so there's a direct URL link to the OEP page
• in release note pages that link directly to a specific feature toggle (eg https://docs.openedx.org/projects/edx-platform/en/latest/references/settings.html#setting-CUSTOM_RESOURCE_TEMPLATES_DIRECTORY) because of the way that page is composed there aren't refs per toggle so it would not be possible to link to a specific toggle any other way.