Additional extension on docs, highlighting internal references

Author: juditnovak

docs/config.rst

@@ -564,14 +564,31 @@ Base options
 
     .. tab:: INI
 
-
        .. code-block:: ini
 
           [testenv]
           set_env = file|conf{/}local.env
                TEST_TIMEOUT = 30
 
 
+    The env file path may include previously defined tox variables:
+
+
+    .. tab:: TOML
+
+       .. code-block:: toml
+
+          [tool.tox.env_run_base]
+          set_env = { file = "{env:variable}" }
+
+    .. tab:: INI
+
+       .. code-block:: ini
+
+          [testenv]
+          set_env = file|{env:variable}
+
+
     .. note::
 
        Environment files are processed using the following rules: