improve and update brainrender installation docs

[alessandrofelder]

Description

What is this PR

• Bug fix
• Addition of a new feature
• Other

Why is this PR needed?

• For the upcoming release of brainrender we move to NEP 29.

What does this PR do?

• Updates and future-proofs the brainrender install docs to the upcoming release
• Mention in the installation docs that some Windows users need pyside2 from conda
• Mention in the installtion docs that videos need ffmpeg installed

References

Closes #106
Closes #102

Partially addresses brainglobe/brainrender#253

How has this PR been tested?

Inspection of locally built website.

Is this a breaking change?

Nope.

Does this PR require an update to the documentation?

NA

Checklist:

• The code has been tested locally (manually)
• [NA] Tests have been added to cover all new functionality (unit & integration)
• [NA] The documentation has been updated to reflect any changes
• The code has been formatted with pre-commit

[adamltyson]

We surely need to install Python right? Might just be easiest to remove this section and just point to the general BG & conda docs.

[alessandrofelder]

Yea I think this is a good idea. One nice thing about the new brainrender is that it's in line with the other BrainGlobe packages now.