building docs instructions miss dependencies

[orbeckst]

Describe the bug
https://userguide.mdanalysis.org/stable/contributing_code.html#working-with-the-code-documentation links to https://userguide.mdanalysis.org/stable/contributing_code.html#id1 for setting up an environment that allows for building of docs.

However, not all necessary packages are include.

When then trying to build docs

python setup.py build_sphinx -E

one gets various errors from sphinx, corresponding to missing packages.

(On a sidenote, IIRC, python setup.py build_sphinx is deprecated and one is encouraged to use the make html in the doc directory.)

Expected behavior
I can copy and paste the build instructions and it works.

Actual behavior
Error messages from sphinx about missing packages.

Additional context

The following are missing (pyproject.toml):

• pybtex
• pybtex-docutils
• sphinxcontrib-bibtex

They can be installed with
conda install pybtex pybtex-docutils sphinxcontrib-bibtex

Note that

• sphinx_rtd_theme
• sphinx-sitemap

can now also be conda-installed (no need for pip anymore).

The with conda installation instructions should then be

# documentation dependencies
conda install sphinx pybtex pybtex-docutils sphinxcontrib-bibtex sphinx_rtd_theme sphinx-sitemap
python -m pip install  msmb_theme==1.2.0

(and remove sphinx from the conda installation command line that comes before the docs)