add online demo URL

[noviluni]

fixes: #882

[codecov]

Codecov Report

Merging #883 (5311acf) into master (615c729) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #883   +/-   ##
=======================================
  Coverage   98.15%   98.15%           
=======================================
  Files         229      229           
  Lines        2495     2495           
=======================================
  Hits         2449     2449           
  Misses         46       46           

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