Add a graph illustrating the build phases to the docs

Visual representations of processes ease the understanding.

doc/_static/diagrams/sphinx_build_phases.dot

@@ -0,0 +1,18 @@
+digraph phases {
+
+    graph [
+        rankdir = LR
+    ]
+
+    node [
+        shape = rect;
+        style = filled;
+        fillcolor ="#f7f7f7";
+        fontcolor = "#0a507a"
+    ]
+
+    Initialization -> Reading;
+    Reading -> "Consistency checks";
+    "Consistency checks" -> Resolving;
+    Resolving -> Writing;
+}

doc/extdev/index.rst

@@ -95,6 +95,9 @@ Build phases
 One thing that is vital in order to understand extension mechanisms is the way
 in which a Sphinx project is built: this works in several phases.
 
+.. graphviz:: /_static/diagrams/sphinx_build_phases.dot
+   :caption: Build phases
+
 **Phase 0: Initialization**
 
 In this phase, almost nothing of interest to us happens.  The source