Documentation: enable building with html builder

[PrimozGodec]

Issue

In #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
• Documentation

[codecov-commenter]

Codecov Report

Merging #121 (71c1a97) into master (3be3086) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #121   +/-   ##
=======================================
  Coverage   82.87%   82.87%           
=======================================
  Files          17       17           
  Lines        2984     2984           
  Branches      350      350           
=======================================
  Hits         2473     2473           
  Misses        458      458           
  Partials       53       53           

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

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