Replace RST section headings with bold text in configuration/index

Section headings (Prometheus & AlertManager, Other, Advanced) were
creating document structure that appeared in the sidebar TOC. Replace
with bold text so only the page title creates structure, keeping the
toctree children properly nested under Send Alerts to Robusta.

https://claude.ai/code/session_0184fwhrjC4ELrYQBMCbyhfS

Author: claude

docs/configuration/index.rst

@@ -5,8 +5,7 @@ Send Alerts to Robusta
 
 Connect your monitoring system to Robusta, to enrich alerts and apply automation rules.
 
-Prometheus & AlertManager
---------------------------
+**Prometheus & AlertManager**
 
 .. grid:: 1 1 2 3
     :gutter: 3
@@ -66,8 +65,7 @@ Prometheus & AlertManager
         :link: alertmanager-integration/victoria-metrics
         :link-type: doc
 
-Other
-------
+**Other**
 
 .. grid:: 1 1 2 3
     :gutter: 3
@@ -102,8 +100,7 @@ Other
         :link: alertmanager-integration/solarwinds
         :link-type: doc
 
-Advanced
----------
+**Advanced**
 
 .. grid:: 1 1 2 3
     :gutter: 3