Remove instruction to pip install Django

[ckartchner]

Since #7063, where Django was added to the install_requires portion of setup.py
doing a separate pip install django has not been required for users of the framework

Today, usage instructions are inconsistent about whether they include that step. For example, the step is already omitted from the README.md Installation instructions, and on the homepage, while still including it elsewhere.

This PR supports the removal of all usage of "pip install Django" from project instructions.

[romainr]

👍

[xordoquy]

I'd rather keep it as it currently is.
If Django installation was indeed unnecessary, it would not need to be added into a new requirement file.
Beside, I'm fine that some step explicitly adds the Django installation step because user may think about adding the constraint on their Django version.