docs(Dropdown): add dedicated example for controlled Dropdown

[layershifter]

Fixes #2249.

[codecov-io]

Codecov Report

Merging #2251 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2251   +/-   ##
=======================================
  Coverage   99.73%   99.73%           
=======================================
  Files         151      151           
  Lines        2624     2624           
=======================================
  Hits         2617     2617           
  Misses          7        7

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[levithomason]

Released in semantic-ui-react@0.76.0