ADD demo images to a new img folder with the documentation site

[ndelangen]

Fixes: #2642

What I did

Add demo images to the documentation site, so we can reference them from anywhere (not just github) (good for npm readme page etc)

[igor-dv]

LGTM, but something is wrong with the docs build

[ndelangen]

I need to do a few more things, I'll merge it when I'm ready

[ndelangen]

Merge when ready

[codecov]

Codecov Report

Merging #2644 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2644   +/-   ##
=======================================
  Coverage   34.34%   34.34%           
=======================================
  Files         389      389           
  Lines        8750     8750           
  Branches      914      918    +4     
=======================================
  Hits         3005     3005           
- Misses       5122     5134   +12     
+ Partials      623      611   -12

Impacted Files	Coverage Δ
app/vue/src/server/config/babel.js	0% <0%> (-100%)	⬇️
app/react/src/server/utils.js	0% <0%> (-53.58%)	⬇️
addons/a11y/src/components/Tabs.js	0% <0%> (ø)	⬆️
addons/viewport/src/components/viewportInfo.js	36.36% <0%> (ø)	⬆️
lib/codemod/src/transforms/move-buildin-addons.js	46.87% <0%> (ø)	⬆️
...react-native/src/manager/components/PreviewHelp.js	0% <0%> (ø)	⬆️
lib/core/src/client/preview/story_store.js	52.88% <0%> (ø)	⬆️
...es__/update-addon-info/update-addon-info.output.js	0% <0%> (ø)	⬆️
lib/ui/src/modules/ui/containers/addon_panel.js	23.52% <0%> (ø)	⬆️
...tive/src/preview/components/StoryListView/index.js	0% <0%> (ø)	⬆️
... and 49 more

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