Docs: Add algorithm diagrams for checkin

doc/development/algorithms/index.rst

@@ -0,0 +1,13 @@
+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