Add documentation for page_config_hook (#469)

* Add documentation for `page_config_hook`

* tweaks

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: jtpio

docs/source/api/handlers.rst

@@ -15,6 +15,32 @@ Module: :mod:`jupyterlab_server.handlers`
 .. autoclass:: LabHandler
     :members:
 
+.. rubric:: Page config hook
+
+``page_config_hook`` is an optional callable you can provide via the Jupyter Server web application settings to customize the page configuration before it is rendered.
+
+- Where: ``web_app.settings['page_config_hook']``
+- Signature: ``callable(handler, page_config) -> dict``
+    - ``handler``: the active ``LabHandler`` instance
+    - ``page_config``: a dict containing the current page configuration
+- Return: the updated page configuration dict.
+
+Example (in a server extension ``load_jupyter_server_extension`` or ``_load_jupyter_server_extension``):
+
+.. code-block:: python
+
+        def my_page_config_hook(handler, page_config: dict) -> dict:
+            page_config.setdefault("extraKeys", {})
+            page_config["extraKeys"]["hello"] = "world"
+            return page_config
+
+
+        def load_jupyter_server_extension(serverapp):
+            web_app = serverapp.web_app
+            web_app.settings["page_config_hook"] = my_page_config_hook
+
+With this hook set, JupyterLab Server will call it during page configuration assembly, letting you inject or tweak values prior to rendering the index page.
+
 .. autofunction:: add_handlers
 
 .. autofunction:: is_url