[docs] add example using EnvVar.get_value() method (#23392)

## Summary & Motivation
Follow up from #23355

Worth documenting because if you try to access a Dagster `EnvVar` without it you'll get an error message that says explicitly "To access the environment variable value, call get_value on the EnvVar"

## How I Tested These Changes
👀

docs/content/guides/dagster/using-environment-variables-and-secrets.mdx

@@ -104,6 +104,14 @@ deployment_name = os.getenv("DAGSTER_CLOUD_DEPLOYMENT_NAME")
 
 Refer to the [Dagster+ Branch Deployments example](#example-2-dagster-branch-deployments) for a real-world example.
 
+You can also call the `get_value()` method on the `EnvVar`:
+
+```python
+from dagster import EnvVar
+
+database_name = EnvVar('DATABASE_NAME').get_value()
+```
+
 ### From Dagster configuration
 
 [Configurable Dagster objects](/concepts/configuration/config-schema) - such as ops, assets, resources, I/O managers, and so on - can accept configuration from environment variables. Dagster provides a native way to specify environment variables in your configuration. These environment variables are retrieved at launch time, rather than on initialization as with `os.getenv`. Refer to the [next section](#using-envvar-vs-osgetenv) for more info.