[Synthetics] Add docs for zip_url

[paulb-elastic]

In the latest version of Synthetics (the beta version that will be included in the 7.13 Heartbeat Docker image), we have added support for zip_url (as an alternative configuration alongside inline and local that are already documented here and here.

These pages should be updated to include details of the new configuration. Here are some details, but let us know if you need more information.

There is a new source.zip_url parameter you can define for a monitor (in place of inline or local). This takes a url parameter for the location of the ZIP file as well as a folder parameter to define where the scripts are, from within the ZIP file.

You can find an example of this here.

A common use case is to point this to a GitHub repository that contains the synthetic test scripts. To limit the downloads, Heartbeat will use the etag to determine if the remote ZIP has been changed before downloading a newer version.

[paulb-elastic]

This has been done in #609