tools/doc-build: Enable default deletion of temporary Python virtual environment

This patch adds the same --keep_venv functionality used in run-checks.py to
doc-build.py, to align the behavior of the two scripts.

Issue-Id: SCM-3553
Signed-off-by: Richard Neill <richard.neill@arm.com>
Change-Id: Id9b4ae7f39f262b65bbb21edbe800346b1d859ae