add docs examples

docs/advanced-usage/views.rst

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+Writing own API endpoint
+========================
+
+The web service loads views dynamically, thus it is possible to add custom API endpoint or even web page. The views must be derived from ``ahriman.web.views.base.BaseView`` and implements HTTP methods. The API specification will be also loaded (if available), but optional. The implementation must be saved into the ``ahriman.web.views`` package
+
+Let's consider example for API endpoint which always returns 204 with no response:
+
+.. code-block:: python
+
+   from aiohttp.web import Response, HTTPNoContent
+
+   from ahriman.web.views.base import BaseView
+
+   class PingView(BaseView):
+
+       async def get(self) -> Response:
+           # do nothing, just raise 204 response
+           # check public methods of the BaseView class for all available controls
+           raise HTTPNoContent
+
+The ``get()`` method can be decorated by ``aiohttp_apispec`` methods, but we will leave it for self-study. Consider checking examples of usages in the main package.
+
+In order to view to be set correctly to routes list, few more options are required to be set. First of all, it is required to specify ``ROUTES`` (list of strings), which contains list of all available routes, e.g.:
+
+.. code-block:: python
+
+   ...
+
+       ROUTES = ["/api/v1/ping"]
+
+In addition, it is also recommended to specify permission level for using this endpoint. Since this endpoint neither does anything nor returns sensitive information, it can be set to ``UserAccess.Unauthorized``:
+
+.. code-block:: python
+
+   ...
+
+       GET_PERMISSION = UserAccess.Unauthorized
+
+That's all. Just save the file as ``/usr/lib/python3.12/site-packages/ahriman/web/views/ping.py`` (replace ``python3.12`` with actual python version) and restart web server.
+
+For more examples and details, please check builtin handlers and classes documentations.