PB-116: pin docutils version

[little-dude]

Summary:

It seems that pip resolves the dependencies by profile. If the [doc]
profile is installed first, docutils ~= 0.16 is picked up, and when
xain-fl's install_requires is resolved, pip sees that docutils is
already installed but has an incompatible version.

I suspect the order in which pip handles the profiles depends on your
environment (pip version, platform, python version, etc.) which is why
we don't see this issue consistently.

As a workaround we explicitely depends on docutils in the [doc]
profile and restrict the version here.

References:

https://xainag.atlassian.net/browse/PB-116

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Reviewer checklist

Reviewer agreement:

• Reviewers assign themselves at the start of the review.
• Reviewers do not commit or merge the merge request.
• Reviewers have to check and mark items in the checklist.

Merge request checklist

• Conforms to the merge request title naming XP-XXX <a description in imperative form>.
• Each commit conforms to the naming convention XP-XXX <a description in imperative form>.
• Linked the ticket in the merge request title or the references section.
• Added an informative merge request summary.

Code checklist

• Conforms to the branch naming XP-XXX-<a_small_stub>.
• Passed scope checks.
• Added or updated tests if needed.
• Added or updated code documentation if needed.
• Conforms to Google docstring style.
• Conforms to XAIN structlog style.