Document monitoring and metrics support

[defagos]

Description

This PR documents monitoring and metrics support.

Changes made

• Add suffixes to articles and extensions to avoid undesired conflicts. Suffixes are not required for tutorials and outlines (tutorial extension).
• Improve metric event naming consistency (metadata / asset). This agrees not only with our general naming but also with conventions we apply in our monitoring JSON exchange format.
• Organize APIs in top-level documentation files.
• Fix SwiftUI extension references for iOS / tvOS 18+. Warnings will be emitted for previous versions but version 18 is around the corner anyway.
• Add default values for metrics tracker configuration parameters.
• Add privacy manifest to the monitoring package.

Checklist

• [x] APIs have been properly documented (if relevant).
• [x] The documentation has been updated (if relevant).
• [x] New unit tests have been written (if relevant).
• [x] The demo has been updated (if relevant).

[defagos]

Thanks for the suggestions!