Add hidden toctree for metric provider child pages

The metric provider sub-pages (external, in-cluster, azure, aws, etc.)
were removed from the root toctree but never added to a toctree on
their parent page. This made them orphan pages that don't appear in the
sidebar navigation. Adding a hidden toctree to metric-providers.rst
restores them in the nav tree.

https://claude.ai/code/session_0184fwhrjC4ELrYQBMCbyhfS

Author: claude

docs/configuration/metric-providers.rst

@@ -5,6 +5,19 @@ General Settings
 
 Connect Robusta to your metrics backend to enable advanced features like alert enrichment with historical data, metric queries, and alert silencing. Metric provider integration is optional but highly recommended.
 
+.. toctree::
+   :maxdepth: 1
+   :hidden:
+
+   metric-providers-in-cluster
+   metric-providers-external
+   metric-providers-azure
+   metric-providers-aws
+   metric-providers-google
+   metric-providers-coralogix
+   metric-providers-victoria
+   metric-providers-grafana-cloud
+
 
 Why Configure a Metric Provider?
 --------------------------------