Add docs for metrics embeds in alerts

What does this MR do?

This adds documentation for metrics charts in the alert detail view. This uses the same behavior under the hood as embedding metrics in incident issues, so the added documentation mostly points to those instructions.

The feature is still in MR !35044 (merged), so this should merge after that MR has.

Related issues:

• #217768 (closed)
• #219181 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team