Add links to ILM docs in agent tutorials and integrations main page

[kilfoyle]

In a review of the ILM docs it was requested that we add links to those docs from various quickstarts, etc.

As part of that, this PR:

• Adds a link at the end of the two standalone agent nginx tutorials:
  

• Adds a tip at the bottom of the main "Manage Elastic Agent Integrations" page.
  

Please see previews:

• Example: Use standalone Elastic Agent with Elastic Cloud Hosted to monitor nginx
• Example: Use standalone Elastic Agent with Elastic Cloud Serverless to monitor nginx
• Manage Elastic Agent Integrations (at the bottom)

Rel: #1568

[github-actions]

🔍 Preview links for changed docs

• reference/fleet/example-standalone-monitor-nginx-serverless.md
• reference/fleet/example-standalone-monitor-nginx.md
• reference/fleet/manage-integrations.md