(Attention: this feature seems to be only available on Google Apps domains not Google Sites created with gmail.com accounts).
The option is available under Manage Site > General
Build add-ons for Google Docs and Sheets - Google Apps Developer Blog
We've just announced Google Docs and Sheets add-ons — new tools created by developers like you that give Google users even more features in
Chromium Blog: WebP improves while rolling out across Google
The WebP team at Google focuses on making the web better through smaller, faster-loading images. We've seen that WebP compares favorably wit
Newsletter Example 3 - Google Apps Script Examples
Google Script Examples has high quality custom libraries to help you get more done by writing less code. This site is maintained by two Goog