Add docstrings to VTK module

[MarcSkovMadsen]

Adds docstrings to VTK panes making them easier to use in a modern IDE like VS Code or PyCharm

[codecov]

Codecov Report

Merging #3369 (1fd2dd9) into master (3504e87) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3369   +/-   ##
=======================================
  Coverage   83.33%   83.34%           
=======================================
  Files         198      198           
  Lines       26791    26791           
=======================================
+ Hits        22327    22329    +2     
+ Misses       4464     4462    -2     

Impacted Files	Coverage Δ
panel/pane/vtk/vtk.py	97.05% <ø> (ø)
panel/io/reload.py	71.73% <0.00%> (+2.17%)	⬆️

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