doc (configuration): update (and rewrite for TOML):

- Configuration is now made with TOML, and the documentation was outdated. It has been
  rewritten for TOML, and autodoc is used to show configuration options available in code
  (and keep it up-to-date).
- New isolation mode is documentated.
- (core/constants): fix the `tmp` isolation mode to inherits the parent configuration like
  `normal`.

libervia/backend/core/constants.py

@@ -131,7 +131,7 @@ class Const:
 
     if ISOLATED_CONFIG_DIR is not None:
         isolated_conf = ISOLATED_CONFIG_DIR / f"{APP_NAME_FILE}.toml"
-        if isolation_mode == "normal":
+        if isolation_mode in ("normal", "tmp"):
             CONFIG_FILES.append(isolated_conf)
         else:
             CONFIG_FILES = [isolated_conf]