tweak documentation a little + change how cms version is defined

[prandla]

i noticed the <title> in the docs still said v1.5.1. so:

• make the docs' version automatically match the cms version
• define the version in pyproject.toml instead of __init__.py (this also allows removing the version detection hack in setup.py)
• show version in sidebar, make sidebar scroll with the page

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 54.68%. Comparing base (18efb50) to head (dc72e80).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1671      +/-   ##
==========================================
+ Coverage   54.67%   54.68%   +0.01%     
==========================================
  Files         336      336              
  Lines       27448    27440       -8     
==========================================
  Hits        15006    15006              
+ Misses      12442    12434       -8     

Flag	Coverage Δ
functionaltests	0.00% <0.00%> (ø)
unittests	54.68% <100.00%> (+0.01%)	⬆️

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