docs: add zone label documentation

[Jeremy Rickards (rickardsjp)]

Description

Adds documentation for the availability-zone label support introduced in #1657. Two docs are updated:

• docs/03-Metrics/modes/advanced.md: documents that source_zone / destination_zone labels can be enabled in remote context by adding zone to sourceLabels and/or destinationLabels, that the value comes from the node's topology.kubernetes.io/zone label, and that unresolvable zones are labelled unknown.
• docs/05-Concepts/CRDs/MetricsConfiguration.md: notes zone as a valid sourceLabels/destinationLabels option and explains the emitted label names and unknown fallback.

This is a documentation-only change; no code is modified.

Related Issue

Documents the feature added in #1657.

Checklist

• I have read the contributing documentation.
• I signed and signed-off the commits (git commit -S -s ...). See this documentation on signing commits.
• I have correctly attributed the author(s) of the code.
• I have tested the changes locally.
• I have followed the project's style guidelines.
• I have updated the documentation, if necessary.
• I have added tests, if applicable.

Screenshots (if applicable) or Testing Completed

Documentation-only change. n/a

Additional Notes

n/a