When installing from Python.org, the `python` command isn't available by
default (in favour of `py`). Support using `py` instead.

There were issues running `hg` when `python` isn't added to the `PATH`,
so the docs are being updated accordingly.
Of course, it's still worth supporting `py`-instead-of-`python`
workflows for `git` users who won't be impacted by that bug.

Differential Revision: https://phabricator.services.mozilla.com/D136286