house-keeping: jsdoc reduce GitHub workload: run only when docs change

this will prevent a large amount of 

deploy: <sha>

to openwrt/gh-pages branches when nothing there has changed

Note: it may not be a bad idea to have docs compile at every commit,
since docs do not change *that* often, and some action or process that
breaks is likely noticed sooner if docs that compile continuously
trigger any broken processes or workflows.