Add local build option

[virtuald]

Omit external URLs. Reference: https://www.chiefdelphi.com/forums/showthread.php?p=1816100

[virtuald]

One thought is to provide an environment variable that can be set, and then modify the urls emitted by the generate script... not sure how to deal with the cross-repo references however.

Another alternative would be to provide a post-processing script that iterates over all the files and changes robotpy.readthedocs.io/ to a relative URL.