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/api.py

@@ -18,6 +18,7 @@ def generate(
     sources: List[Union[GallerySource, Dict[str, Any]]] = None,
     clean_first: bool = False,
     verbose: bool = False,
+    source_to_update: GallerySource = None,
 ) -> bool:
     """
     Generate a scientific gallery from plot sources.
@@ -33,8 +34,13 @@ def generate(
                    Required if config is not provided.
         sources: List of GallerySource objects or dicts.
                 Required if config is not provided.
-        clean_first: If True, removes and recreates the gallery directory
-        verbose: If True, prints progress messages
+        clean_first: If True, removes and recreates the gallery directory.
+                    If False (default), performs incremental update.
+        verbose: If True, prints progress messages.
+        source_to_update: Optional specific source to update. When provided,
+                         only this source is regenerated (incremental mode).
+                         Other sources in config are preserved in the index.
+                         Only effective when clean_first is False.
 
     Returns:
         True if gallery generation was successful, False otherwise
@@ -116,6 +122,23 @@ def generate(
                 if verbose:
                     print(f"Warning: Could not clean directory: {e}")
                 return False
+        elif source_to_update and gallery_root.exists():
+            # Incremental mode: only clean the specific source subdirectory
+            source_subdir = gallery_root / source_to_update.name
+            if source_subdir.exists():
+                if verbose:
+                    print(
+                        f"Updating source directory {source_to_update.name}..."
+                    )
+                try:
+                    shutil.rmtree(source_subdir)
+                except Exception as e:
+                    if verbose:
+                        print(
+                            f"Warning: Could not clean source subdirectory "
+                            f"{source_subdir}: {e}"
+                        )
+                    return False
 
         try:
             gallery_root.mkdir(parents=True, exist_ok=True)
@@ -141,6 +164,14 @@ def generate(
         # Process sources
         source_subdirs = []
         for source in config.sources:
+            # Skip sources not matching the update target (if specified)
+            if source_to_update and source.name != source_to_update.name:
+                # Still include them in the index if they exist
+                source_web_dir = gallery_root / source.name
+                if source_web_dir.exists():
+                    source_subdirs.append(source.name)
+                continue
+
             try:
                 source_path = Path(source.path).resolve()