docs: add plugins docs (#1319)

add plugin docs

Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

docs/concepts/plugins.md

@@ -0,0 +1,101 @@
+Docling allows to be extended with third-party plugins which extend the choice of options provided in several steps of the pipeline.
+
+Plugins are loaded via the [pluggy](https://github.com/pytest-dev/pluggy/) system which allows third-party developers to register the new capabilities using the [setuptools entrypoint](https://setuptools.pypa.io/en/latest/userguide/entry_point.html#entry-points-for-plugins).
+
+The actual entrypoint definition might vary, depending on the packaging system you are using. Here are a few examples:
+
+=== "pyproject.toml"
+
+    ```toml
+    [project.entry-points."docling"]
+    your_plugin_name = "your_package.module"
+    ```
+
+=== "poetry v1 pyproject.toml"
+
+    ```toml
+    [tool.poetry.plugins."docling"]
+    your_plugin_name = "your_package.module"
+    ```
+
+=== "setup.cfg"
+
+    ```ini
+    [options.entry_points]
+    docling =
+        your_plugin_name = your_package.module
+    ```
+
+=== "setup.py"
+
+    ```py
+    from setuptools import setup
+
+    setup(
+        # ...,
+        entry_points = {
+            'docling': [
+                'your_plugin_name = "your_package.module"'
+            ]
+        }
+    )
+    ```
+
+- `your_plugin_name` is the name you choose for your plugin. This must be unique among the broader Docling ecosystem.
+- `your_package.module` is the reference to the module in your package which is responsible for the plugin registration.
+
+## Plugin factories
+
+### OCR factory
+
+The OCR factory allows to provide more OCR engines to the Docling users.
+
+The content of `your_package.module` registers the OCR engines with a code similar to:
+
+```py
+# Factory registration
+def ocr_engines():
+    return {
+        "ocr_engines": [
+            YourOcrModel,
+        ]
+    }
+```
+
+where `YourOcrModel` must implement the [`BaseOcrModel`](https://github.com/docling-project/docling/blob/main/docling/models/base_ocr_model.py#L23) and provide an options class derived from [`OcrOptions`](https://github.com/docling-project/docling/blob/main/docling/datamodel/pipeline_options.py#L105).
+
+If you look for an example, the [default Docling plugins](https://github.com/docling-project/docling/blob/main/docling/models/plugins/defaults.py) is a good starting point.
+
+## Third-party plugins
+
+When the plugin is not provided by the main `docling` package but by a third-party package this have to be enabled explicitly via the `allow_external_plugins` option.
+
+```py
+from docling.datamodel.base_models import InputFormat
+from docling.datamodel.pipeline_options import PdfPipelineOptions
+from docling.document_converter import DocumentConverter, PdfFormatOption
+
+pipeline_options = PdfPipelineOptions()
+pipeline_options.allow_external_plugins = True  # <-- enabled the external plugins
+pipeline_options.ocr_options = YourOptions  # <-- your options here
+
+doc_converter = DocumentConverter(
+    format_options={
+        InputFormat.PDF: PdfFormatOption(
+            pipeline_options=pipeline_options
+        )
+    }
+)
+```
+
+### Using the `docling` CLI
+
+Similarly, when using the `docling` users have to enable external plugins before selecting the new one.
+
+```sh
+# Show the external plugins
+docling --show-external-plugins
+
+# Run docling with the new plugin
+docling --allow-external-plugins --ocr-engine=NAME
+```

mkdocs.yml

@@ -67,6 +67,7 @@ nav:
     - Architecture: concepts/architecture.md
     - Docling Document: concepts/docling_document.md
     - Chunking: concepts/chunking.md
+    - Plugins: concepts/plugins.md
   - Examples:
     - Examples: examples/index.md
     - 🔀 Conversion: