remove docs requirements from dev requirments

[davidism]

Unfortunately, pinning Sphinx with pip-compile causes Jinja to be pinned as well, which then overwrites the editable install unless it's done afterwards. And adding -e in the requirements file causes Dependabot to fail. It doesn't seem like that's going to be solved by pip (pypa/pip#8307), pip-compile (jazzband/pip-tools#1150), or Dependabot (dependabot/feedback#936) any time soon, so for now I'm just going to remove the docs requirements from the dev requirements.

Alternatively could have left them in and required doing pip install -e . after pip install -r requirements/dev.txt, but I figured just removing it would be less error prone. If users need to build the docs locally, they can do pip install -r requirements/doc.in to get the direct dependencies without the sub-dependency pins. Read the Docs will still use the pinned file because it can install local Jinja after.

fixes #1215