docs: add succinct Gomplate datasource example (#2866)

* docs: add succinct Gomplate datasource example

Consolidates two duplicate open PRs (#2748, #2651) that both attempted
to answer issue #2650 with a much longer AWS-specific example. Uses
Erik's succinct rewrite from his review comment on #2748 instead.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* docs: link !include, fix describe command in datasource example

Address CodeRabbit review feedback on #2866: the example command was
missing --query '.vars' to match the shown output. Also link !include
to its function docs per review feedback.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>
Co-authored-by: Andriy Knysh <aknysh@users.noreply.github.com>

Author: 3 people

website/docs/templates/datasources.mdx

@@ -246,6 +246,62 @@ components:
 ```
 </File>
 
+## Example: Using a Gomplate Datasource
+
+<PillBox>Advanced</PillBox>
+
+Gomplate datasources make external data available inside template expressions.
+
+> **Note:** Atmos [`!include`](/functions/yaml/include) functions and Gomplate datasources both load external data, but they serve different purposes. [`!include`](/functions/yaml/include) imports data directly into the Atmos configuration as part of YAML processing. Gomplate datasources expose data within template expressions, where it can be transformed, filtered, combined, or otherwise manipulated using Gomplate functions before being assigned to a value.
+
+Configure a YAML datasource:
+
+<File title="atmos.yaml">
+
+```yaml
+settings:
+  templates:
+    settings:
+      gomplate:
+        datasources:
+          config:
+            url: "file://config.yaml"
+```
+
+</File>
+
+Use the datasource in template expressions:
+
+<File title="stack.yaml">
+
+```yaml
+components:
+  terraform:
+    app:
+      vars:
+        account_id: '{{ (datasource "config").account_id }}'
+        region: '{{ (datasource "config").region }}'
+```
+
+</File>
+
+Render the final configuration:
+
+<Terminal>
+
+```bash
+atmos describe component app -s dev --query '.vars'
+```
+
+</Terminal>
+
+Example output:
+
+```yaml
+account_id: "111111111111"
+region: "eu-west-2"
+```
+
 ## Using templates in the URLs of `datasources`
 
 <PillBox>Advanced</PillBox>