PTFM-9656 Improve hygiene of Python doc builds.

Some state is kept in _sources that would cause the "source" pages to
not be refreshed.
PTFM-9656 Add a docstring for DXFile.get_download_url.
PTFM-9656 More doc build hygiene.

(Nuke the build products from orbit... it's the only way to be sure.)
Fix broken autodoc links found by the scanner
PTFM-7846 Add docs for DXDataObject dynamic attributes.
Changelog and dxR version update for v0.70.1