Update installation page descriptions

- Clarify that Robusta works with any observability stack (DataDog, NewRelic, Prometheus, SolarWinds, etc.)
- Update Robusta + Prometheus description to highlight it's for new environments without existing observability

Author: claude

docs/setup-robusta/installation/index.rst

@@ -23,11 +23,11 @@ Installation Guides
         :link: standalone-installation
         :link-type: doc
 
-        Use Robusta's AI Agent alongside DataDog, NewRelic, SolarWinds, and more.
+        Use Robusta's AI Agent alongside any observability stack - DataDog, NewRelic, Prometheus, SolarWinds, and more.
 
     .. grid-item-card:: Install Robusta + Prometheus
         :class-card: sd-bg-text-light
         :link: all-in-one-installation
         :link-type: doc
 
-        Five minute setup. Great default alerts. Powered by Prometheus and Robusta. 
+        For brand new environments without existing observability. Five minute setup. Great default alerts.