[Synthetics] Document infrastructure monitoring for k8s

[andrewvc]

While the Elastic Synthetics app is NOT intended for infrastructure monitoring of k8s etc. this is a FAQ. We should have a page mentioning that users should not do this, and instead recommend they use heartbeat with autodiscovery OR standalone agent with autodiscovery IF they don't need hints based autodiscovery.

[bmorelli25]

When would you like to see this change? Is this something you'll lead or you'd like a tech writer to lead?

[paulb-elastic]

Crosslinking with #1847

[bmorelli25]

Going through issues that have been open for a bit. This seems like a very easy enhancement (if it's still necessary).

[eedugon]

@emilioalvap , @bturquet , do you know if this is still relevant and an improvement we should make to the docs? In such case I could take care of it soon.

[emilioalvap]

Syncing offline with @eedugon