dependencies: Migrate docs deps to root directory
This is a first step in migrating all project dependencies to Pipenv.
This commit specifically adds the docs dependencies into a new Pipfile
specified at the root of the project.

Migrate all project dependencies to Pipfile
This commit removes the multiple requirements.txt files used to install
and manage dependencies and opts in favor of the Pipfile managed by
Pipenv:
    https://github.com/pypa/pipfile
    https://pipenv.org/
The purpose of this change is to maintain DRY principles. This makes our
dependencies easier to manage by managing them in one place and only
dividing them by standard dependencies or development dependencies.
Future changes after this commit will include documentation improvements
and updates to the Dockerfile to install dependencies in the Pipfile
format.

Refactor Dockerfile, use pipenv for Python packages
This commit refactors the Dockerfile to use fewer layers when building
the image. It also re-arranges the order of how things are run to
benefit some steps like installing Python dependencies.
Ultimately, this commit leverages Pipenv for managing and installing
dependencies within the project. This is one additional change to
improving awareness of dependencies and ensuring development,
production, and staging will always use the same set of dependencies.

Dockerfile: Install development dependencies
This is not sustainable and not recommended. But in practice, this is
the same as we were doing before. I wonder if it's better to have a prod
and dev Dockerfile with different Django settings and Pipenv flags.