TOC Generation broken

[kreuzberger]

The generation of the toc seems not be only broken with newer sphinx versions (see #112).
Also with earlier versions the links in the toc point to invalid pages cause they are refering a span element BEFORE <h2> and then have the link on the page before.

To improve this handling i tried to implement a test framework for it with some basic tests, that fail on documents using a TOC.

Feel free to discuss it, i will spend a little time on it the next weeks before christmas.
Maybe we get a stable solution here for the project.

sphinx-simplepdf is besides sphinx-needs one of the most important repos in the useblocks universe, cause most of the generated documents are delivered as PDF generated from HTML, cause i need to have BOTH output formats in my productive builds.

[kreuzberger]

The toc generation works, but the default handling in the main.css is not good.
The hx elements should lead to an page-break, but the links in the toc entries refer to the span element before the section before the hx Elements.
Fix:
Find the span element in toctree_fix and add specific classes to adapt page break behaviour.
E.g. if custom_css is given with

div.body h1{
    display: none;
}

h2 {
    page-break-before: avoid !important;
    break-before:avoid !important;
}

span.anchor-before-heading {
    display: block;
    page-break-before: always;
    height: 0;
    margin: 0;
    padding: 0;
}

span.anchor-before-heading.first {
    page-break-before: avoid !important;
}

the toc links work like expected for toc entries. but not for <h2> in the main document.