- venv rule is now conditional, and only does anything if $VENVDIR does not exist
- add rule "clean-venv"
(cherry picked from commit d22c876d5a)
Co-authored-by: Jack DeVries <58614260+jdevries3133@users.noreply.github.com>
In the documentation, the `env` directory is specified when we execute
the `make venv` command. But in the code, `make venv` will create the
virtualenv inside the `venv` directory (defined by `VENVDIR`)
Rather than requiring the path to blurb and/or sphinx-build to be specified to the make rule, enhance the Doc/Makefile to look for each first in a virtual environment created by make venv and, if not found, look on the normal process PATH. This allows the Doc/Makefile to take advantage of an installed spinx-build or blurb and, thus, do the right thing most of the time. Also, make the directory for the venv be configurable and document the `make venv` target.
