add explicit npm ci in prepare-docs

scripts/prepare-docs.sh

@@ -2,6 +2,8 @@
 
 OUTDIR=docs/modules/api/pages/
 
+npm ci
+
 rm -rf "$OUTDIR"
 solidity-docgen -t docs -o "$OUTDIR" -e contracts/mocks,contracts/examples
 node scripts/gen-nav.js "$OUTDIR" > "$OUTDIR/../nav.adoc"