[Synthetics] Adjust private location docs to support base elastic-agent

[emilioalvap]

Summary

Adjust synthetics docs to support elastic-agent base image for non-browser monitors.

Relates to https://github.com/elastic/synthetics-dev/issues/402.

[github-actions]

🔍 Preview links for changed docs

• reference/fleet/elastic-agent-container.md
• solutions/observability/synthetics/migrate-from-elastic-synthetics-integration.md
• solutions/observability/synthetics/monitor-resources-on-private-networks.md
• solutions/observability/synthetics/support-matrix.md

[lucabelluccini]

I've made some suggestions as I think being more explicit as possible is the best.
Also I would recommend, for the 'default' path, to show the complete option.

There are less chances someone will have to redo the download/tutorial because they blindly copy pasted the steps without reading the warnings and then they discover they would like to run a browser monitor.

[emilioalvap]

Thanks for the suggestions @lucabelluccini, I still need to add a reference to alternative elastic-agent (basic) install methods, which we will also be supporting from now on. I'll update the default tab options to show the complete variant first as you suggested

[mdbirnstiehl]

I left a few suggestions. Let me know if you have any questions or comments.

[colleenmcginnis]

I left a couple minor suggestions below to make the way we refer to synthetic browser monitors in the Fleet docs more consistent with how we refer to them in the Observability docs.

[github-actions]

Vale Linting Results

Summary: 1 suggestion found

💡 Suggestions (1)

File	Line	Rule	Message
solutions/observability/synthetics/monitor-resources-on-private-networks.md	61	Elastic.Acronyms	'ICMP' has no definition.

[emilioalvap]

Thanks @colleenmcginnis and @mdbirnstiehl for the suggestions! Please review again and let me know if this is ready for merging.

[emilioalvap]

@colleenmcginnis @mdbirnstiehl Kind reminder to review the committed changes and provide an approval if everything is Ok