Add READTHEDOCS_DATA for default version URL

[agjohnson]

Raised in readthedocs/doc-diff#3, it would be helpful to add a value for the base URL that the diff library will use to compare pages. My current approach is hacky and full of holes.

I'm not certain, but I think what I'm looking for is at least the hostname of the default version, though with changes around subproject URL structure/etc, I guess this might need to be the hostname and base path to the project (https://docs.readthedocs.io/projects/foo/en/latest/).

Ultimately, this needs to be the full URL and path to the page to compare (https://docs.readthedocs.io/en/latest/some/path/page.html). I'm currently grabbing the page name from the READTHEDOCS_DATA page attribute, which might not be ideal. Currently I just append .html to the URL, and this likely doesn't work for all projects.

So, this might be a resolved base URL, or fully resolved base URL and file path name. Either probably work for the diff library use case.

I'm guessing this would be implemented in readthedocs-sphinx-ext, but I'm also assuming we'll want to use our resolver for this value.

[ericholscher]

You might be able to use this? https://github.com/readthedocs/readthedocs.org/blob/main/readthedocs/api/v2/views/core_views.py#L23-L27

[agjohnson]

Ah yes, that might be enough!

[humitos]

We are removing the usage of READTHEDOCS_DATA and migrating all of it to the new API endpoint, /readthedocs-config/ which will be used by the new js client. Developers can subscribe to a js event to access this data as well: readthedocs/addons#60

Related to readthedocs/addons#23 also; where we are building the docdiff integration.