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.

.github/workflows/jsdoc.yml

@@ -4,6 +4,12 @@ on:
   push:
     branches:
       - master
+    paths:
+      # One day we might include all htdocs folders:
+      # - '**/htdocs/**'
+      # Until then, follow jsdoc.conf.json:source directive
+      - 'modules/luci-base/htdocs/luci-static/resources/**'
+      - 'docs/**'
 
 jobs:
   deploy: