Simplify conf.py and add video reqs

[IsabelBirds]

No description provided.

[IsabelBirds]

@robredpath I'm trying to get the video reqs sorted, following IATI/iati-docs-base#8
I'm getting this error locally:

Extension error:
Could not import extension sphinxcontrib.video (exception: No module named 'sphinxcontrib.video')
Sphinx exited with exit code: 2

Could you have at look for what I've missed? Thanks!

[robredpath]

@IsabelBirds this works for me!

I usually rebuild the container (cmd+shift+P -> Dev Containers: Rebuild Container) after doing anything with python versions or requirements to ensure I've got a clean build; when working locally the python module installation doesn't happen every time you make a change (whereas on RTD it does).