Include or link to shiboken2 docs from the doc-snapshot

The shiboken docs live in:

<virtualenvname>_build/<build_version>_release/shiboken2/doc/html

Additionally, there is some dependencies related to this documentation that cross reference the documentation generated under:
<virtualenvname>_build/<build_version>_release/shiboken2/ApiExtractor/doc/html

Both projects are independent sphinx projects, and the shiboken one uses the ApiExtractor docs via intersphinx, so ApiExtractor must be built first.

They should be moved to the doc-snapshot server along with the pyside2 docs and linked to from the main index page.