[3.6] bpo-30964: Mention ensurepip in package installation docs (GH-2797)

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

(cherry picked from commit b3527bfefd7a0188d43a2d7515ac6addd97a8202)
2 files changed