Fix documentation previews in pull requests (#2015)

* migrate to openzeppelin-docs-preview script

* update netlify configuration

* update docs-preview-script dependency

* remove old docs directory from gitignore

* update oz-docs script for live reload

* update oz-docs scripts to latest

* replace child_process.execFileSync with spawnSync

* update oz-docs-preview

scripts/prepare-docs.sh

@@ -1,8 +1,12 @@
 #!/usr/bin/env bash
 
+set -o errexit
+
 OUTDIR=docs/modules/api/pages/
 
-npm ci
+if [ ! -d node_modules ]; then
+  npm ci
+fi
 
 rm -rf "$OUTDIR"
 solidity-docgen -t docs -o "$OUTDIR" -e contracts/mocks,contracts/examples