Fixes some redirection links in docs.

Adds a new 'Pip not installed' section that covers
running `ensurepip` manually, and also references
the relevant section of the Python Packaging User
Guide.

As part of the update, the documentation was updated to normalize
around the term "virtual environment" instead of relying too heavily
on "venv" for the same meaning and leading to inconsistent usage of
either.

Thanks to Steve Piercy for the patch.