Automatically detect a staging SiteGround site

We have this function already `Jetpack::is_staging_site` in [this code](https://github.com/Automattic/jetpack/blob/1dd3187b6d522efbf13cb994471fb223d1d91315/class.jetpack.php#L5835)

Now, we just need to put a constant or URL in the `$known_staging` array.

* SiteGround info: https://www.siteground.co.uk/tutorials/staging/
* Our doc: https://jetpack.com/support/staging-sites/

Ticket: 695508-zen

Open class.jetpack.php at Jetpack::is_staging_site and inspect the $known_staging array. Review the linked SiteGround staging information and add the appropriate constant or URL so SiteGround staging sites are detected; confirm the change matches the staging-sites documentation.

Written by the indexing model from the issue text.