This makes it possible to change the branch of the documentation that
URLs are pointing to without having to modify all class reference
files.
In the XML class reference, the `$DOCS_URL` placeholder should be used,
and will be replaced automatically in the editor and when generating
the RST class reference.
The documentation branch string is set in `version.py`.
Co-authored-by: Hugo Locurcio <hugo.locurcio@hugo.pro>
