Revised `make docs` command fails in GitLab CI because no local Python env is used

Seeing this in the latest pipeline run after merging !42 (merged), which added .venv/bin/ to the mkdocs command and uncommented make docs in the GitLab CI steps:

make: .venv/bin/mkdocs: No such file or directory
make: *** [Makefile:55: site/index.html] Error 127

This is because the GitLab CI script doesn't use a local .venv, unlike the Makefile.