sphinx-build is passed a blanket "." to build all docs. However, if a
virtual environment is placed within the docs directory, sphinx will try
to build it which will fail due to some weird files it has.

This excludes the most common virtual environment directories from the
build to prevent this.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: Ieeb14cfc5730d21c986611feb0ed379c58dfcae2