docs: env vars in source queries and pages

sites/docs/pages/reference/cli.md

@@ -71,4 +71,46 @@ N.B. Environment variables are **case sensitive**, so you should preserve the ca
 
 ### .env Files
 
-Evidence will read in environment variables from a `.env` file in the root of your project. This is useful for local development.
+Evidence will read in environment variables from a `.env` file in the root of your project. This is useful for local development.
+
+### Environment Variables in Source Queries
+
+Environment variables to be used in source queries should be prefixed with `EVIDENCE_VAR__` (note the double underscore). They can be used in source queries with `${EVIDENCE_VAR__variable_name}`.
+
+```
+EVIDENCE_VAR__customer_name="Acme Corporation"
+```
+
+```
+select *
+from orders
+where customer_name = '${customer_name}'
+```
+
+The quotes would be omitted if the variable was not a string.
+
+### Environment Variables in Pages
+
+Environment variables to be used in pages should be prefixed with `VITE_`. They can be accessed with `import.meta.env.VITE_variable_name`.
+
+`.env`
+```bash
+VITE_customer_attribute=premium
+```
+
+`index.md`
+```html
+<script>
+  const customer_attribute = import.meta.env.VITE_customer_attribute;
+</script>
+
+{#if customer_attribute === 'premium'}
+
+premium
+
+{:else if customer_attribute === 'free'}
+
+free
+
+{/if}
+```