Add support for rtd documentation

[PrimozGodec]

Issue

Currently users can reach widget documentation from Orange

Description of changes

Add support for at least online documentation

Includes

• [X] Code changes
• [ ] Tests
• [ ] Documentation

[codecov-commenter]

Codecov Report

Merging #387 (140c50d) into master (2f78bb8) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #387   +/-   ##
=======================================
  Coverage   80.49%   80.49%           
=======================================
  Files          21       21           
  Lines        5450     5450           
=======================================
  Hits         4387     4387           
  Misses       1063     1063           

Impacted Files	Coverage Δ
orangecontrib/single_cell/widgets/__init__.py	100.00% <ø> (ø)

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