[Elastic Agent] Update diagnostics structure and document how to disable output status reporting

[cmacknz]

Summary

We have introduced beats receivers and now use the OpenTelemetry collector to perform self-monitoring by default in 9.2.1. See https://github.com/elastic/elastic-agent/blob/main/docs/hybrid-agent-beats-receivers.md.

This has brought in a few changes the public documentation should include:

1. There are several new files in the diagnostics structure to document. Probably the formatting of this could be improved but I stuck with what was already there.
2. The output for inputs that use beat receivers will now report themselves as degraded or failed if there is a connectivity issue. This can be turned off in the output settings. The main reason to do this is because there is no output status UI yet and/or users may find false health changes annoying.

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes
• No

[github-actions]

Vale Linting Results

Summary: 5 suggestions found

💡 Suggestions (5)

File	Line	Rule	Message
reference/fleet/agent-command-reference.md	67	Elastic.WordChoice	Consider using 'refer to (if it's a document), view (if it's a UI element)' instead of 'see', unless the term is in the UI.
reference/fleet/elasticsearch-output.md	236	Elastic.WordChoice	Consider using 'deactivated, deselected, hidden, turned off, unavailable' instead of 'disabled', unless the term is in the UI.
reference/fleet/elasticsearch-output.md	236	Elastic.FutureTense	'will not' might be in future tense. Write in the present tense to describe the state of the product as it is now.
reference/fleet/es-output-settings.md	136	Elastic.WordChoice	Consider using 'deactivated, deselected, hidden, turned off, unavailable' instead of 'disabled', unless the term is in the UI.
reference/fleet/es-output-settings.md	136	Elastic.FutureTense	'will not' might be in future tense. Write in the present tense to describe the state of the product as it is now.

[github-actions]

🔍 Preview links for changed docs

• reference/fleet/agent-command-reference.md
• reference/fleet/elasticsearch-output.md
• reference/fleet/es-output-settings.md