Update installation documentation for Robusta (#1999)

* 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

* Add explicit step to enable Prometheus stack in generated_values.yaml

Users installing Robusta + Prometheus now have a clear step to add
enablePrometheusStack: true to their generated_values.yaml file.

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: aantn

docs/setup-robusta/installation/all-in-one-installation.rst

@@ -19,6 +19,15 @@ Prerequisites
    :header_update_levels:
    :file: setup-robusta/installation/_generate_config.jinja
 
+Enable Prometheus Stack
+------------------------------
+
+Add the following to your ``generated_values.yaml`` file:
+
+.. code-block:: yaml
+
+    enablePrometheusStack: true
+
 .. include:: ./_helm_install_with_prometheus.inc.rst
 
 Next Steps

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.