Add 7.16 release notes for Elastic Agent and Fleet

[dedemorton]

Adds release notes to the Elastic Agent and Fleet Guide.

Preview link: https://observability-docs_1319.docs-preview.app.elstc.co/guide/en/fleet/master/release-notes-7.16.0.html

Reviewers: Please respond to these questions:

• Do we have any breaking changes or known issues that we want to document in 7.16?
• The following PRs seem like internals (stuff users don't need to know). Please confirm whether we want to document these changes in the release notes:
  [elastic-agent] proxy requests to subprocesses to their metrics endpoints beats#28165
  Allow HTTP metrics to run in bootstrap mode. Add ability to adjust timeouts for Fleet Server. beats#28260
• This seems like internals (stuff users might want to know), but I’m not sure how to describe this for users. Do we want to document this in the release notes? What is the user impact of this PR?
  [Elastic Agent] Add ability to communicate with Kibana through service token beats#28096

NOTE: The diff includes a template for the next update. You can ignore everything after after:

// ---------------------
//TEMPLATE

Closes #1319

[apmmachine]

A documentation preview will be available soon:

• 📚 HTML diff
• 📘 Fleet and Elastic Agent Guide
• 📙 Observability Guide

[mostlyjason]

Worth including this one elastic/kibana#111002?

[mostlyjason]

Not sure if you are tracking integrations UI here, but this was a big one elastic/kibana#93084

[jlind23]

PRs are internal indeed, we do not need to communicate about it.
For the 7.16 breaking change/deprecation there is the custom beat generator deprecation, don't know if it has to be mentionned.
Related issue: elastic/beats#28426

[dedemorton]

Ok, I've added a comment to the Beats BC PR to track the generator deprecation: elastic/beats#29300

[dedemorton]

Not sure if you are tracking integrations UI here, but this was a big one elastic/kibana#93084

I think that should be covered in the Kibana docs instead of here.

[dedemorton]

Can I get approval to merge this since the release is going out today?

[dedemorton]

@elasticmachine run elasticsearch-ci/docs skiplinkcheck