Install npm dependencies in the docs deploy workflow

build-docs now runs scripts/gen-patch-schema.mjs before jsonnet, which
imports markdown-patch-2 from node_modules — but this workflow predates
that prerequisite and never ran npm ci, so the 5.0.0 tag's docs deploy
failed with ERR_MODULE_NOT_FOUND. Adds the same Node 22 setup the test
workflow uses, with npm caching.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>