Add support for incremental updates in gallery generation

- Introduced `source_to_update` parameter to selectively regenerate specific sources.
- Updated documentation to reflect new behavior for `clean_first` and `source_to_update`.
- Modified CLI and legacy CLI to handle source overrides appropriately.

gallery/cli.py

@@ -68,7 +68,9 @@ Examples:
         config = GalleryConfig.from_yaml(args.config)
 
         # Handle source override
+        source_to_update = None
         if args.source:
+            from gallery.config import GallerySource
             source_path = Path(args.source).resolve()
 
             # Check if source is in config
@@ -80,24 +82,26 @@ Examples:
 
             # If not in config, create a temporary source entry
             if matching_source is None:
-                from gallery.config import GallerySource
                 source_name = source_path.name
-                config.sources = [
-                    GallerySource(name=source_name, path=source_path)
-                ]
+                source_to_update = GallerySource(
+                    name=source_name, 
+                    path=source_path
+                )
+                config.sources.append(source_to_update)
                 if args.verbose:
                     print(
                         f"Source {args.source} not in config. "
                         f"Adding temporarily as '{source_name}'"
                     )
             else:
-                config.sources = [matching_source]
+                source_to_update = matching_source
 
         # Generate gallery
         success = generate(
             config=config,
             clean_first=args.clean,
-            verbose=args.verbose
+            verbose=args.verbose,
+            source_to_update=source_to_update
         )
 
         sys.exit(0 if success else 1)