Documentation: enable building with html builder

[PrimozGodec]

Issue

In https://github.com/biolab/orange3-educational/pull/109 we changed the documentation theme to shpinx rtd theme. It was found out that it does not support htmlhelp builder.

Description of changes

Since with this theme, it is not required to build the documentation with htmlhelp I am changing the setting such that they are compatible with html builder. From now on we build documentation with make html instead of make htmlhelp

Includes

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

[codecov-commenter]

Codecov Report

Merging #237 (d00281e) into master (c9db4f6) will increase coverage by 0.04%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #237      +/-   ##
==========================================
+ Coverage   73.29%   73.33%   +0.04%     
==========================================
  Files          21       21              
  Lines        2973     2974       +1     
==========================================
+ Hits         2179     2181       +2     
+ Misses        794      793       -1     

Impacted Files	Coverage Δ
orangecontrib/network/widgets/__init__.py	94.44% <ø> (ø)
orangecontrib/network/widgets/graphview.py	67.03% <0.00%> (-0.25%)	:arrow_down:
orangecontrib/network/network/community.py	60.63% <0.00%> (+2.12%)	:arrow_up:

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