Document Real Browser Synthetic Integrations

[paulb-elastic]

The documentations that covers the current (as of 7.13) Synthetic Integration capabilities (https://github.com/elastic/observability-docs/blob/master/docs/en/ingest-management/getting-started.asciidoc#step-4-monitor-http-tcp-and-icmp-services) needs to be updated to add the Real Browser capabilities that are being added to the Integration in elastic/uptime#276

This documentation need to make it clear that Real Browser integrations can only run on Elastic Agents that are running from a specific Docker image (i.e. they will not work on the binary (non Docker) installation or the standard Docker Elastic Agent installation). The specific Docker version will include the Elastic Agent, plus Synthetics and required dependencies.

[paulb-elastic]

Shifting this to 7.15, as this won't be going in, in 7.14 now