[DOC] Add flake8-docstrings and adapt the docstring and typing to it

[tobiasah]

Description:
Add https://pypi.org/project/flake8-docstrings/ to the lint step.

Also fix some typehints from #136

[codecov]

Codecov Report

Merging #159 (992510d) into main (3a51c81) will decrease coverage by 0.00%.
The diff coverage is 97.22%.

@@            Coverage Diff             @@
##             main     #159      +/-   ##
==========================================
- Coverage   98.83%   98.82%   -0.01%     
==========================================
  Files          34       34              
  Lines        2233     2130     -103     
==========================================
- Hits         2207     2105     -102     
+ Misses         26       25       -1     

Impacted Files	Coverage Δ
src/zhinst/toolkit/command_table.py	98.79% <ø> (ø)
src/zhinst/toolkit/driver/devices/hdawg.py	100.00% <ø> (ø)
src/zhinst/toolkit/driver/devices/shfqa.py	97.95% <ø> (ø)
src/zhinst/toolkit/driver/devices/shfsg.py	98.03% <ø> (ø)
src/zhinst/toolkit/driver/modules/__init__.py	100.00% <ø> (ø)
...rc/zhinst/toolkit/driver/modules/sweeper_module.py	95.00% <ø> (ø)
src/zhinst/toolkit/driver/nodes/shfqa_scope.py	100.00% <ø> (ø)
src/zhinst/toolkit/driver/nodes/spectroscopy.py	100.00% <ø> (ø)
src/zhinst/toolkit/interface.py	100.00% <ø> (ø)
src/zhinst/toolkit/nodetree/node.py	99.30% <ø> (ø)
... and 16 more

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