Add NIM dashboard and docs preview message (#19239)

* Add NIM dashboard and docs preview message

* Apply suggestions from code review

Co-authored-by: Sandra (neko) <165049174+neko-dd@users.noreply.github.com>

---------

Co-authored-by: Sandra (neko) <165049174+neko-dd@users.noreply.github.com>

Author: Kyle-Neale

nvidia_nim/README.md

@@ -6,6 +6,10 @@ This check monitors [NVIDIA NIM][1] through the Datadog Agent.
 
 ## Setup
 
+<div class="alert alert-warning">
+This integration is currently in Preview. Its availability is subject to change in the future. 
+</div>
+
 Follow the instructions below to install and configure this check for an Agent running on a host. For containerized environments, see the [Autodiscovery Integration Templates][3] for guidance on applying these instructions.
 
 **Requirements**:

nvidia_nim/assets/dashboards/nvidia_nim_overview.json

@@ -75,7 +75,7 @@
                         "id": 4528647613111842,
                         "definition": {
                             "type": "note",
-                            "content": "Here you can see an overview of your LLM of your system activity and any NIM alerts. The service checks on the left speak to the health of your NVIDIA NIM environment, while the ones on the right report on the readiness of your dependencies.\n",
+                            "content": "This integration is currently in Preview. Its availability is subject to change in the future. \n\nHere you can see an overview of your LLM of your system activity and any NIM alerts. The service checks on the left speak to the health of your NVIDIA NIM environment, and the checks on the right report on the readiness of your dependencies.",
                             "background_color": "green",
                             "font_size": "14",
                             "text_align": "center",