[DOC] add architecture notes on rendering pipeline

doc/architecture/rendering.md

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+# 🦉 Rendering Pipeline 🦉
+
+We explain here how Owl is designed, from the perspective of its rendering
+pipeline.
+
+Warning: these notes are technical by nature, and intended for people working
+on Owl (or interested in understanding its design).
+
+## Overview
+
+A rendering occurs in two phases:
+
+- virtual rendering: this generates the virtual dom in memory, asynchronously
+- patch: applies a virtual tree to the screen (synchronously)
+
+
+There are several classes involved in a rendering:
+
+- components
+- a scheduler
+- fibers: small objects containing some metadata, associated with a rendering of
+  a specific component
+
+
+Components are organized in a dynamic component tree, visible in the user
+interface.  Whenever a rendering is initiated in a component `C`:
+- a fiber is created on `C` with the rendering props information
+- the virtual rendering phase starts on C (will asynchronously render all the
+  child components)
+- the fiber is added to the scheduler, which will poll continuously, every
+  animation frame, if the fiber is done
+- once it is done, the scheduler will call the task callback, which will apply
+  the patch (if it was not cancelled in the meantime).
+