Fix documentation previews in pull requests (OpenZeppelin#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

Author: frangio

.gitignore

@@ -43,7 +43,6 @@ build/
 
 # docs artifacts
 docs/modules/api
-openzeppelin-docs
 
 # only used to package @openzeppelin/contracts
 contracts/build/

netlify.toml

@@ -1,3 +1,3 @@
 [build]
-command = "npm run docs build"
-publish = "openzeppelin-docs/build/site"
+command = "npm run docs"
+publish = "build/site"