Add versions plugin to docs

[panosvoudouris]

Issue:

What I did

Added the versions plugin to the list of addons.

How to test

Nothing to test.

[codecov]

Codecov Report

Merging #1269 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1269   +/-   ##
=======================================
  Coverage   13.73%   13.73%           
=======================================
  Files         207      207           
  Lines        4638     4638           
  Branches      518      504   -14     
=======================================
  Hits          637      637           
- Misses       3545     3567   +22     
+ Partials      456      434   -22

Impacted Files	Coverage Δ
app/react/src/server/iframe.html.js	0% <0%> (ø)	⬆️
addons/knobs/src/components/types/Select.js	7.93% <0%> (ø)	⬆️
addons/info/src/components/PropTable.js	0% <0%> (ø)	⬆️
addons/info/src/components/markdown/htags.js	0% <0%> (ø)	⬆️
app/react/src/client/preview/config_api.js	0% <0%> (ø)	⬆️
addons/knobs/src/KnobStore.js	6.81% <0%> (ø)	⬆️
app/react/src/client/preview/client_api.js	39.28% <0%> (ø)	⬆️
.../ui/src/modules/ui/components/layout/dimensions.js	15.62% <0%> (ø)	⬆️
app/react-native/src/bin/storybook-start.js	0% <0%> (ø)	⬆️
lib/ui/src/modules/ui/containers/layout.js	12.5% <0%> (ø)	⬆️
... and 18 more

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[shilman]

Great addon, thanks!! 🎖