Add a config user guide and improve the documentation

Add module-level documentation to the `frequenz.sdk.config` module and
include it in the generated documentation's user guide.

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

docs/user-guide/config.md

@@ -0,0 +1,9 @@
+# Configuration
+
+::: frequenz.sdk.config
+    options:
+        members: []
+        show_bases: false
+        show_root_heading: false
+        show_root_toc_entry: false
+        show_source: false