CI Fix: separating docs dependencies from requirements.txt

[GarethCabournDavies]

Recent CI jobs have been failing the docker build because it looks like the docs dependencies are clashing with the prerequisites needed by the docker build.

Instead of manually uninstalling them, which does work, I've had a look at separating the docs dependencies so that they aren't used when building the docker image.

We can run pip install ".[docs]" - same way as we do with [test] or [igwn], and get the docs stuff in.

Standard information about the request

This is a: bug fix / prerequisites separation
This change fixes: the CI, and docker build

This change: has appropriate unit tests, follows style guidelines (See e.g. PEP8), has been proposed using the contribution guidelines

Motivation

Fix the CI, make things a bit more robust

Contents

• Move the requirements for the docs into setup.py as an extra_requires for [docs]
• Make it so that pixi actually loads that

Testing performed

If the CI passes for this PR, then we are good. It has passed on my fork, so should pass here as well

• The author of this pull request confirms they will adhere to the code of conduct

[spxiwh]

@GarethCabournDavies Look over #5391 ... It looks like pypi has made another release, and the underlying issue has still not been fixed. As this could still raise issues in other places we should continue with the strategy in 5391 of either avoiding pypi releases with this issue or going with the other fix there (see commit history)