build: Merge doc linting into 'npm test'

build: Merge doc linting into 'npm test'

Whether JSDuck or JSDoc3, it's good to verify that there are no
regressions in the doc syntax. This has been enforced by WMF CI
for many years with a dedicated Jenkins job.

However, both 'grunt lint' and 'npm run doc' take a relatively
small amount of time in CI:

* grunt lint: ~ 35s (not incl 'npm install')
* npm run doc: ~ 10s (not incl 'npm install')

Change-Id: If22b7bc64266e43088c7dec8138d81c938687fb9