Missing documentation for override variables on custom themes

[stsewd]

Details

There isn't documentation about integrating a custom theme with rdt (see related issue #3515). I'm not sure if you like to keep supporting this on the long way anyway. But if so, we need to document how to override some values of READTHEDOCS_DATA https://github.com/rtfd/readthedocs.org/blob/master/readthedocs/core/static-src/core/js/doc-embed/rtd-data.js#L54

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stsewd]

I think this is more for internal usage rather than exposing it to users, if anything this should be configurable via the theme.