Change documentation theme to sphinx_rtd_theme

[PrimozGodec]

While ago we discussed that we will use the same documentation theme for add-ons. After some discussion, we decided on sphinx-rtd-theme. The alternative is Alabaster but its sidebar is not so readable and it breaks long addon names in two lines.

With this PR I am implementing the sphinx-rtd-theme. I also changed that the menu bar collapses on the medium screen already (before it collapsed on small screens). On the medium screen, the content column was narrow already and it works well for widget documentation in Orange since at the default Orange help window size the sidebar is collapsed.

[PrimozGodec]

https://github.com/biolab/orange3-imageanalytics/pull/201 should fix tests

[codecov[bot]]

Codecov Report

Merging #200 (f81700a) into master (5fea616) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #200   +/-   ##
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  Coverage   67.75%   67.75%           
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  Files          15       15           
  Lines        2810     2810           
  Branches      402      402           
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  Hits         1904     1904           
  Misses        790      790           
  Partials      116      116