category.py: select doc subpage based on family and site code

List convention for Template documentation subpage in family file and
retrieve it from there when needed.

Implement doc_subpage as Site property.
Implemented tests in site_tests.py.
Fixed some pep257 in site_tests.py.

Bug: 55017

Change-Id: I0b9b44484e2ac390ab23d1d0242adad519ff0e22
Merge "category.py: select doc subpage based on family and site code"