Better docs for redirects

[stsewd]

https://docs.readthedocs.io/en/latest/user-defined-redirects.html#redirect-types doesn't mention that Page redirect and Exact redirect can redirect to an external URL, not just to a rtd page #4008 (comment).

Also, the Exact redirect has an undocumented feature

https://github.com/rtfd/readthedocs.org/blob/e36955f737b810cbda7a07326b933f8cf43bb698/readthedocs/redirects/models.py#L154-L159

Where you can use $rest on the from url to do a redirection similar to Prefix redirect

From url: /dev/v1/$rest
To url: /en/latest/

docs.readthedocs.io/dev/v1/install.html -> docs.readthedocs.io/en/latest/install.html