Support for Sphinx Book Theme

[twodrops]

The Book Theme is a well maintained Sphinx-Theme in comparision to the popular Sphinx RTD Theme. (66 open PRs as on 05.09.2022).

Similar to the RTD Theme, the Book Theme does not work with Sphinx-Needs 1.0.1+ because of doc-utils problem, which hopefully will be fixed by them soon. An issue exists for this already.

The Book Theme however has some issues with Sphinx-Needs 0.7.9 and this problem would probably exist even when the docutils issue above is solved. So as a preparation, these issues needs to be solved.

1. The rendering of Sphinx-Needs with Book Theme is not ideal. Grey Needs Layout (Alabaster like) which does not go well with the overall Book Theme look and feel.
2. The Collapse/Expand Button is missing.

Needs-Table rendering works pretty good with Sphinx Book Theme.

[danwos]

This should be checked also the https://pydata-sphinx-theme.readthedocs.io/en/stable/ theme.

However, changing the common need-style is CSS work.
We can't change it in general, as then it would maybe not fit other themes anymore.

But we have this concept of need-themes, by default, modern, blank, and dark are available.
https://pydata-sphinx-theme.readthedocs.io/en/stable/
Maybe one of them is already working.
For sure we could add a fourth one, which fits best for book/pydata theme.

[twodrops]

ah, sorry I completely forgot about https://sphinxcontrib-needs.readthedocs.io/en/latest/configuration.html#needs-css
Therefore it makes sense to leave it to the user to customize it to match the theme used. We could close this.
Thanks for the hint.