Adding documentation on how to enable agent config in inventory

[hush-hush]

What does this PR do?

We should hold until DASH to merge this PR. The related feature will be announce during our conference.

Motivation

Additional Notes

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Reviewer checklist

• [ ] Review the changed files.
• [ ] Review the URLs listed in the Preview section.
• [ ] Check images for PII
• [ ] Review any mentions of "Contact Datadog support" for internal support documentation.

[github-actions[bot]]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/maxime/inventory-agent-config/agent/guide/collecting_agent_configuration

Modified Files

• https://docs-staging.datadoghq.com/maxime/inventory-agent-config/agent/guide/

[hestonhoffman]

I've consolidated the info in this PR into #15539, so I'm going to close this one and we can move any remaining discussion over there.