Repo Link missing in readthedocs theme if custom repo_name used

The readthedocs theme is missing a fallback option for the repo link if `repo_name` is not set to one of 3 predefined providers (GitHub, BitBucket, or GitLab).

It would make sense if the `repo_name` has been customised to display the custom name with a generic git icon (eg. [this one](https://fontawesome.com/icons/git-alt?f=brands&s=solid))