Pin sphinx<8 / update sphinx config for sphinx8

[dweindl]

Some dependency lifted there sphinx<8 requirement, but this leads to a number of issues with other dependencies. Therefore, require sphinx<8 for now.

Anyways fix some upcoming issue with sphinx 8 when doc2path will return something path-like (see ICB-DCM/pyPESTO#1482).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.77%. Comparing base (cd56633) to head (7d6578b).
Report is 86 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2532   +/-   ##
========================================
  Coverage    77.77%   77.77%           
========================================
  Files          325      325           
  Lines        21048    21048           
  Branches      1472     1472           
========================================
  Hits         16371    16371           
  Misses        4650     4650           
  Partials        27       27           

Flag	Coverage Δ
cpp	73.49% <ø> (ø)
cpp_python	34.03% <ø> (ø)
petab	37.04% <ø> (ø)
python	72.33% <ø> (ø)

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[socket-security]

New and removed dependencies detected. Learn more about Socket for GitHub ↗︎

Package	New capabilities	Transitives	Size	Publisher
pypi/[email protected]	environment, eval, filesystem, network, shell, unsafe	+24	450 MB	AA-Turner, tk0miya

🚮 Removed packages: pypi/[email protected]

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