Update plugin option schema validation code sample (gatsbyjs#28904)

* Update configuring-usage-with-plugin-options.md

The testPluginOptionsSchema returns promise.

* Add mention about Kontent implementation

Author: gatsbybot

yarn.lock

@@ -1761,12 +1761,7 @@
     "@types/node" "*"
     form-data "^3.0.0"
 
-"@types/node@*":
-  version "14.14.7"
-  resolved "https://registry.yarnpkg.com/@types/node/-/node-14.14.7.tgz#8ea1e8f8eae2430cf440564b98c6dfce1ec5945d"
-  integrity sha512-Zw1vhUSQZYw+7u5dAwNbIA9TuTotpzY/OF7sJM9FqPOF3SPjKnxrjoTktXDZgUjybf4cWVBP7O8wvKdSaGHweg==
-
-"@types/node@^8.5.7":
+"@types/node@*", "@types/node@^8.5.7":
   version "8.10.66"
   resolved "https://registry.yarnpkg.com/@types/node/-/node-8.10.66.tgz#dd035d409df322acc83dff62a602f12a5783bbb3"
   integrity sha512-tktOkFUA4kXx2hhhrB8bIFb5TbwzS4uOhKEmwiD+NoiL0qtP2OQ9mFldbgD4dV1djrlBYP6eBuQZiWjuHUpqFw==