[QA] Publishing OOUI demos on doc.wikimedia.org

[Legoktm]

Hi,

Last week James F and I (with some help from Krinkle) worked on
publishing the OOUI demos on doc.wikimedia.org[1].

We added a "npm run-script demos" command to OOUI[2], which builds a
self-contained demos/ directory that contains all of the JS and PHP
dependencies, which is then copied to doc.wm.o[3].

There is now a 'npm-run-demos' macro (and 'npm-run-demos' job) which
other projects can use.

[1] https://doc.wikimedia.org/oojs-ui/master/demos/
[2] https://gerrit.wikimedia.org/r/#/c/222529/
[3] https://gerrit.wikimedia.org/r/#/c/222530/

-- Legoktm