Docs: Add algorithm diagrams for checkin

doc/development/algorithms/checkin.rst

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+.. spelling: libpretixsync
+
+Check-in algorithms
+===================
+
+When a ticket is scanned at the entrance or exit of an event, we follow a series of steps to determine whether
+the check-in is allowed or not. To understand some of the terms in the following diagrams, you should also check
+out the documentation of the :ref:`ticket redemption API endpoint <rest-checkin-redeem>`.
+
+Server-side
+-----------
+
+The following diagram shows the series of checks executed on the server when a ticket is redeemed through the API.
+Some simplifications have been made, for example the de-duplication mechanism based on the ``nonce`` parameter
+to prevent re-uploads of the same scan is not shown.
+
+.. image:: /images/checkin_online.png
+
+Client-side
+-----------
+
+The process of verifying tickets offline is a little different. There are two different approaches,
+depending on whether we have information about all tickets in the local database. The following diagram shows
+the algorithm as currently implemented in recent versions of `libpretixsync`_.
+
+.. image:: /images/checkin_offline.png
+
+.. _libpretixsync: https://github.com/pretix/libpretixsync

doc/development/algorithms/index.rst

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+Algorithms
+==========
+
+The business logic inside pretix is full of complex algorithms making decisions based on all the hundreds of settings
+and input parameters available. Some of them are documented here as graphs, either because fully understanding them is very
+when working on features close to them, or because they also need to be re-implemented by client-side components like our
+ticket scanning apps and we want to ensure the implementations are as similar as possible to avoid confusion.
+
+.. toctree::
+   :maxdepth: 2
+
+   checkin
+   layouts

doc/development/algorithms/layouts.rst

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+.. spelling: pretixPOS
+
+Ticket layout
+=============
+
+When a ticket is exported to PDF, the system needs to decide which of multiple PDF layouts to use. The
+following diagram shows the steps of the decision, showing both the implementation in pretix itself as
+well as the implementation in `pretixPOS`_.
+
+The process can be influenced by plugins, which is demonstrated with the example of the shipping plugin.
+
+.. image:: /images/ticket_layouts.png
+
+
+.. _pretixPOS: https://pretix.eu/about/en/pos