Fix documentation build on readthedocs

[andreasabel]

Lifted from #7770 (comment):

Locally the docs build fine with make users-guide, but:

@fgaz wrote:

I triggered a readthedocs build on master and it failed

A documentation concerning the offending section_self_link is here:
https://docutils.sourceforge.io/docs/user/config.html#section-self-link
It says:

New in Docutils 0.18.

Locally, I get 0.16:

docutils>=0.12 in ./.python-sphinx-virtualenv/lib/python3.9/site-packages (from sphinx==3.1.*->-r doc/requirements.txt (line 1)) (0.16)

Remotely (on rtd), 0.18 is used:

Requirement already satisfied: docutils>=0.12 in /home/docs/checkouts/readthedocs.org/user_builds/cabal/envs/7770/lib/python3.7/site-packages (from sphinx==3.1.*->-r doc/requirements.txt (line 1)) (0.18)

That can make the difference.
Likely, our requirements.txt is not watertight, so we might get packages that do not work together.