Add support for rediracting to external urls

[TheTripleV]

Pros:

• Allows for documentation that has moved to another domain

Cons:

• Allows for circumventing of cycle checking and destination checking

[phlax]

related to this it would be kinda useful to be able to redirect to externals specified by intersphinx - not sure how it would work exactly as currently the redirects are based on the .rst path rather than rst anchors

i can open a separate ticket if its useful to do so

[TheTripleV]

Nice idea. Could you make a new issue for it?