bpo-44756: in ./Doc, `make build` depends on `make html` (GH-27403) (GH-27410)

- venv rule is now conditional, and only does anything if $VENVDIR does not exist
- add rule "clean-venv"
(cherry picked from commit d22c876d5ac5fa464337d2e82654b8d87a83cb1b)

Co-authored-by: Jack DeVries <58614260+jdevries3133@users.noreply.github.com>