prepare release 0.25

[janfb]

• add changelog
• bump version
• refactor docs instructions

fixes #1645

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@0c75b9c). Learn more about missing BASE report.
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1643   +/-   ##
=======================================
  Coverage        ?   82.49%           
=======================================
  Files           ?      134           
  Lines           ?    11137           
  Branches        ?        0           
=======================================
  Hits            ?     9188           
  Misses          ?     1949           
  Partials        ?        0           

Flag	Coverage Δ
unittests	82.49% <100.00%> (?)

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Files with missing lines	Coverage Δ
sbi/__version__.py	100.00% <100.00%> (ø)