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

Assuming that the `MOZILLABUILD` environment variable is set, allow
invoking Mach from non-MozillaBuild terminals.

Note that MozillaBuild still needs to be installed, and the
`MOZILLABUILD` environment variable will have to be set.

For future reference: when I tried setting this up with Windows
Store's Python 3.9, I encountered issues when running binaries installed
via `pip`: it would fail with `abort: failed to load Python DLL
python3x.dll`.

Differential Revision: https://phabricator.services.mozilla.com/D133936