chore(ExApp): migrate and split AppAPI documentation to relevant manuals

Signed-off-by: Edward Ly <contact@edward.ly>

developer_manual/exapp_development/notes_for_developers/ExAppDevelopmentSteps.rst

@@ -0,0 +1,124 @@
+.. _ExAppDevelopment:
+
+ExApp development
+=================
+
+ExApp development process is similar to the development of the regular Nextcloud PHP app,
+and should follow the same guidelines in terms of security, design and coding style (`ref <https://docs.nextcloud.com/server/latest/developer_manual/getting_started/development_process.html>`_)
+based on your programming language standards.
+
+Despite the fact, that ExApp can be developed in any language, it's still recommended to have the understanding
+of the Nextcloud `PHP request life cycle <https://docs.nextcloud.com/server/latest/developer_manual/basics/request_lifecycle.html>`_ and other basic concepts,
+as they are usually similar to the ExApp backend to which Nextcloud communicates.
+
+You can think about ExApp like a microservice (Docker container)
+that runs separately from Nextcloud on the Deploy daemon, which can be remote or local.
+The communication between Nextcloud and ExApp is done via network secured with AppAPI authentication (:ref:`app_api_auth`).
+
+Lets go through the ExApp development steps briefly.
+
+0. Setting up the development environment
+-----------------------------------------
+
+First of all, you need to have a Nextcloud dev setup, refer to :ref:`DevelopmentEnvironment` for more details.
+
+1. Starting from template
+-------------------------
+
+Next step, is to setup the ExApp skeleton.
+There are several ExApp examples available so you can have a look at them and start from.
+The ExApp template and examples:
+
+	- ``[Python]`` `App Skeleton <https://github.com/cloud-py-api/app-skeleton-python>`_
+	- ``[Python]`` `UI Example Skeleton <https://github.com/cloud-py-api/ui_example>`_
+	- ``[Python]`` `More complex ExApp UI example with 3rdparty service <https://github.com/cloud-py-api/visionatrix>`_
+	- ``[GoLang]`` `Go Lang ExApp example <https://github.com/cloud-py-api/file_to_text_example>`_
+	- etc.
+
+They contain the basic structure of the ExApp, including:
+
+- Dockerfile
+- ExApp backend
+- ExApp frontend
+- Manual translations tools setup and example script to convert translation files to Nextcloud l10n format and for your programming language
+- Some necessary GitHub workflows (e.g. `Docker image build workflows <https://github.com/cloud-py-api/visionatrix/tree/main/.github/workflows>`_)
+
+More details are available in the :ref:`ExAppOverview` section.
+
+
+3. Development
+--------------
+
+The development process basically contains from the following steps:
+
+- Implement the ExApp <-> Nextcloud :ref:`lifecycle methods <ex_app_lifecycle_methods>`:
+	#. ``/heartbeat``: ExApp heartbeat method.
+	#. ``/init``: ExApp initialization method.
+	#. ``/enabled``: ExApp enable/disable method.
+- Implement the ExApp backend API and logic.
+- Implement the ExApp frontend (Nextcloud Vue.js app) [optional].
+
+
+4. Packaging
+------------
+
+The ExApp packaging can be done manually, or via GitHub actions.
+It is recommended to use GitHub actions for packaging,
+as it will automate the process of building the Docker image and pushing it to the Docker registry.
+The GitHub action workflow for building Docker images can be found in the `3rdparty service example <https://github.com/cloud-py-api/visionatrix>`_.
+
+4.1 Hardware acceleration
+*************************
+
+If your ExApp work with GPUs, you should consider building different Docker images for each compute device.
+Currently, there are 3 main compute devices to target with custom Docker images:
+
+- CPU (default, no specific tag)
+- GPU: CUDA (NVIDIA) (``<image_name>:<version>-cuda``)
+- GPU: ROCm (AMD) (``<image_name>:<version>-rocm``)
+
+.. note::
+
+	If the Deploy daemon configured with the GPU compute device,
+	AppAPI will try to pull the Docker image with the GPU support first (``<image_name>:<version>-<cuda|rocm>``, `ref PR <https://github.com/cloud-py-api/app_api/pull/340>`_).
+	If the image is not found, AppAPI will try to pull the base (CPU) image (``<image_name>:<version>``).
+
+
+Dockerfile
+**********
+
+The Dockerfile is required to build the Docker image for the ExApp.
+The guideline for writing the Dockerfile can be found in the `Dockerfile best practices <https://docs.docker.com/develop/develop-images/dockerfile_best-practices/>`_.
+
+Short recommendations:
+
+1. Keep the Dockerfile as small as possible.
+2. Use minimal base images to keep the image size small.
+3. Place rarely changing instructions at the top of the Dockerfile to take advantage of Docker's caching mechanism.
+
+
+Logging
+*******
+
+The logs in Docker container are shown from the standard output and error streams.
+To be able admin to see the important logs from ExApp container,
+you should consider redirecting the logs to the standard output and standard error streams.
+More info in `official docs for logging <https://docs.docker.com/config/containers/logging/>`_.
+
+
+5. AppStore publishing
+----------------------
+
+Once the ExApp is ready, and the Docker image is available in the Docker registry,
+you can follow `the AppStore publishing process <https://nextcloudappstore.readthedocs.io/en/latest/developer.html>`_.
+It's the same as for the regular Nextcloud app, but with the requirement of :ref:`the ExApp specific fields <ex_app_info_xml_metadata>` in the ``appinfo/info.xml`` file.
+
+
+6. Testing
+----------
+
+It is important to ensure that your ExApp works as expected.
+We recommend to have different types of dev setup configuration to test all of them.
+While the main development is done locally via ``manual_install``, you must also ensure that
+the ExApp works correctly in Docker container with http and https Docker Socket proxy.
+