Improve requirement handling

Change summary:

• The requirements are no longer in each documentation folder. They are now shared between both folders. I see no reason to keep them separate for now, if we ever have different dependencies we can change this again.
• There is now two separate requirements files: requirements.txt contains the desired packages, and requirements.lock contains the actual packages and specific versions that have been tested. Installing dependencies should always be done from requirements.lock. To upgrade dependencies, we simply need to create a fresh Python 3 virtual environments, install from requirements.txt into it, and run pip freeze > requirements.lock.
• I’ve updated the documentation for contributors to encourage using a Python 3 virtual environment instead of a system-wide or user-wide installation.

Note: If there is currently any process that automatically builds and deploys the documentation, it will need to be changed to install dependencies from the new requirements.lock file in the root folder.