Document optional dependencies

[Midnighter]

• [x] fix #69
• [x] description of feature/fix

[jfy133]

Does the list of extras for development include the extra packages needed for rendering the website?

[Midnighter]

Does the list of extras for development include the extra packages needed for rendering the website?

Good point, they don't. Those are separately in docs/requirements.txt. Let's see how we can do this. 🤔

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (c557970) 82.10% compared to head (33debbb) 82.10%.

:exclamation: Current head 33debbb differs from pull request most recent head 1973838. Consider uploading reports for the commit 1973838 to get more accurate results

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Additional details and impacted files

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[Midnighter]

I've added another extra docs for those dependencies. So at least it's within the same system but for now separate from the pure dev dependencies. I did this only because historically I've been keeping them apart. They could easily be combined, however.