docs/config: Document where to set default templates

The user may want to know how to change the default template used. To
help this, add documentation about how to change this on the configs.

docs/config.md

@@ -299,6 +299,24 @@ revsets.log = "main@origin.."
 The default value for `revsets.log` is
 `'present(@) | ancestors(immutable_heads().., 2) | present(trunk())'`.
 
+### Default Template
+
+You can configure the template used when no `-T` is specified.
+
+- `templates.log` for `jj log`
+- `templates.op_log` for `jj op log`
+- `templates.show` for `jj show`
+
+```toml
+[templates]
+# Use builtin log template
+log = "builtin_log_compact"
+# Use builtin op log template
+op_log = "builtin_log_compact"
+# Use builtin show template
+show = "builtin_log_detailed"
+```
+
 ### Graph style
 
 ```toml