docs: Document virtualenv for docs project

Summary

The Development Environment document for the Documentation project suggests running pip to install the needed dependencies. However, starting with Python 3.11, it's not possible to run plain pip install because it fails with the following error:

error: externally-managed-environment

with the recommendation to use virtualenv or pipx. See this StackOverflow answer for more details about this change.

To conform to this requirement, the Development Environment document has been extended to cover creating and activating virtualenv.

Location

https://docs.flybywiresim.com/dev-corner/development-projects/documentation-project/documentation/#required-tools

Discord username (if different from GitHub): xmudrii

flybywiresim / docs

docs: Document virtualenv for docs project #989

Summary

Location