[singlehtml] add docname to section anchor to make them unique

[gastmaier]

Purpose

Follow up to #13717, inverting the logic, instead of patching the toctree to yield "#id1" instead of "#document-path/to#id1", have the section id to be docname preffixed, solving non-unique ids in singlehtml.
Allows to remove post Sphinx transforms like in here

Top level overview of current behavior

• ID collision is resolved per doc (#already-used -> #id1, #already-used -> #id2).
• There is no ID collision resolution on singlehtml step.

Approach taken

Based on the LaTeX builder solution.
sphinx/writers/latex.py#hypertarget[withdoc=True] method suffixes docutils id with the docname.
In my implementation I edit ids['0'] directly to not have to overwrite the whole visit_section method, but I understand if requested to not modify the tree and instead overwrite.

On the format #document-test/extra#id1

It is compatible with HTML anchoring, CSS and JavaScript selectors, but require escaping:

#document-test\/extra\#test {color: #f00;}

document.querySelector('#document-test\\/extra\\#test')

Tests

The following tests are relevant:

• tests/test_builders/test_build_html_tocdepth.py
• test_build_html_numfig.py

References

• [singlehtml] toctree no filename with anchor #13717 (replaces)
• 🐛 Fix singlehtml target uris to be same-document references #11970,
• singlehtml: Use same-document hyperlinks for internal project references #12551
• singlehtml: deprecate the 'fix_refuris' helper function #13037