feat: fill in outline for new docs

docs/concepts/hooks-extension-framework.rst

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+Extending Open edX with the Hooks Extensions Framework
+######################################################
+
+To sustain the growth of the Open edX ecosystem, the business rules of the
+platform must be open for extension following the open-closed principle. This
+framework allows developers to do just that without needing to fork and modify
+the main edx-platform repository.
+
+Context
+*******
+
+Hooks are predefined places in the edx-platform core where externally defined
+functions can take place. In some cases, those functions can alter what the user
+sees or experiences in the platform. Other cases are informative only. All cases
+are meant to be extended using Open edX plugins and configuration.
+
+Hooks can be of two types, events and filters. Events are in essence signals, in
+that they are sent in specific application places and whose listeners can extend
+functionality. On the other hand Filters are passed data and can act on it
+before this data is put back in the original application flow. In order to allow
+extension developers to use the Events and Filters definitions on their plugins,
+both kinds of hooks are defined in lightweight external libraries.
+
+* `openedx-filters`_
+* `openedx-events`_
+
+Hooks are designed with stability in mind. The main goal is that developers can
+use them to change the functionality of the platform as needed and still be able
+to migrate to newer open releases with very little to no development effort. In
+the case of filters, this is detailed in the `naming and versioning ADR`_.
+
+A longer description of the framework and it's history can be found in `OEP 50`_.
+
+.. _OEP 50: https://open-edx-proposals.readthedocs.io/en/latest/oep-0050-hooks-extension-framework.html
+.. _naming and versioning ADR: https://github.com/eduNEXT/openedx-events/blob/main/docs/decisions/0002-events-naming-and-versioning.rst
+.. _openedx-filters: https://github.com/eduNEXT/openedx-filters
+.. _openedx-events: https://github.com/eduNEXT/openedx-events
+
+On the technical side events are implemented through django signals which makes
+them run in the same python process as the lms or cms. Furthermore, events block
+the running process. Listeners of an event are encouraged to monitor the
+performance or use alternative arch patterns such as receiving the event and
+defer to launching async tasks than do the slow processing.
+
+On the other hand, filters are implemented using a pipeline mechanism, that executes
+a list of functions called ``steps`` configured through Django settings. Each
+pipeline step receives a dictionary with data, process it and returns an output. During
+this process, they can alter the application execution flow by halting the process
+or modifying their input arguments.

docs/concepts/index.rst

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 Concepts
-########
+========
+
+.. toctree::
+   :maxdepth: 1
+   :caption: Contents:
+
+    hooks-extension-framework

docs/how-tos/create-new-filter.rst

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+Create a new filter
+###################
+
+Let's say you want to modify the behavior of the students enrollment. You want
+at least the following use cases:
+
+
+
+
+Context
+*******

docs/how-tos/index.rst

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-How-tos
-#######
+Concepts
+========
+
+.. toctree::
+   :maxdepth: 1
+   :caption: Contents:
+
+    create-new-filter
+    triggering-filters
+    configure-filter
+    filter-unittest