Docs: add predefined macro vars to jinja macros page (#3766)

Author: treysp

docs/concepts/macros/jinja_macros.md

@@ -50,6 +50,30 @@ JINJA_STATEMENT_BEGIN;
 JINJA_END;
 ```
 
+## SQLMesh predefined variables
+
+SQLMesh provides multiple [predefined macro variables](./macro_variables.md) you may reference in jinja code.
+
+Some predefined variables provide information about the SQLMesh project itself, like the [`runtime_stage`](./macro_variables.md#runtime-variables) and [`this_model`](./macro_variables.md#runtime-variables) variables.
+
+Other predefined variables are [temporal](./macro_variables.md#temporal-variables), like `start_ds` and `execution_date`. They are used to build incremental model queries and are only available in incremental model kinds.
+
+Access predefined macro variables by passing their unquoted name in curly braces. For example, this demonstrates how to access the `start_ds` and `end_ds` variables:
+
+```sql linenums="1"
+JINJA_QUERY_BEGIN;
+
+SELECT *
+FROM table
+WHERE time_column BETWEEN '{{ start_ds }}' and '{{ end_ds }}';
+
+JINJA_END;
+```
+
+Because the two macro variables return string values, we must surround the curly braces with single quotes `'`. Other macro variables, such as `start_epoch`, return numeric values and do not require the single quotes.
+
+Unlike other predefined macro variables, the `gateway` variable name cannot be unquoted. Instead, it must use the `{{ var() }}` function like [user-defined variables](#global-variables) do: `{{ var('gateway') }}`.
+
 ## User-defined variables
 
 SQLMesh supports two kinds of user-defined macro variables: global and local.