The attribute documentation description is more of a
GPR2 language standpoint of the attribute value whereas
we have a whole chapter about its functional use.
We need to link one to the other to be complete about
the attribute documentation.
Issue: eng/gpr/gpr#41
* move the python module to a separate folder from metadata
* adjust a bit the phrasing in conf.py
* split attributes doc generation from the doc build
* fix gprdoc --display default value
* output of gprdoc is now put in the build directory
* document attributes inheritance
* fix index descriptions
* gprdoc test now outputs only differences between expected doc and
commited one.
TN: V324-004
Change-Id: Id4c152b189a20a53f2df653328ac1da4cd3ec4e7
