Combine requirements-dev.txt and docs/requirements.txt

[shaunagm]

Currently we have three requirements files, the standard one in requirements.txt, a requirements-dev.txt in the top level, and another requirements.txt in the docs folder. The latter has requirements for building docs, and requirements-dev.txt has requirements for testing and linting. There's no reason not to combine the latter two (probably makes sense to remove the requirements.txt in the docs folder).

When doing this, make sure to check our CI workflows - presumably this changes how the docs are automatically built, for instance. We also want to check any of our documentation, like the Contributing Guide, and update instructions there as well.

[rdhyee]

I'd like to tackle this issue. Next step is for me to write down what @shaunagm told me over Zoom.

I'm editing this note the next morning to say that now that I've looked at Shauna's comment above that I don't have anything more to write down. I'll have to dig into the issue to have anything substantive to say.

[shaunagm]

Note: this comment talks about some relevant CI configuration here: #1122 (comment)

Although right now the CI assumes all dev requirements are in requirements-dev.txt in the main level, and ignores the requirements.txt in the docs repository (I believe!) so it doesn't actually need to be changed when we move docs/requirements.txt to requirements-dev.txt. We'd just be bringing the files in line with the CI. (We still need to double-check the docs build though)