[DOC] move reference doc in subfolder

doc/architecture/vdom.md

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+# 🦉 VDom 🦉
+
+Owl is a declarative component system: we declare the structure of the component
+tree, and Owl will translate that to a list of imperative operations. This
+translation is done by a virtual dom. This is the low level layer of Owl, most
+developer will not need to call directly the virtual dom functions.
+
+The main idea behind a virtual dom is to keep a in-memory representation of the
+DOM (called a virtual node), and whenever some change is needed, to regenerate
+a new representation, compute the difference between the old and the new, then
+apply the changes.
+
+`vdom` exports two functions:
+
+- `h`: create a new virtual node
+- `patch`: compare two virtual nodes, and apply the difference.
+
+Note: Owl's virtual dom is a fork of [snabbdom](https://github.com/snabbdom/snabbdom).