WIP merging synth phases with example

Replace `typical_phases.rst` and `examples.rst` with a single `example_synth.rst`.
Also updating the counter example to match.

Aims to reduce redundancy, and simplify the getting started section.
Details on things like `proc`, `memory` and `fsm` should instead be in the advanced section (under the new `synth` subsection).

docs/source/using_yosys/index.rst

@@ -4,6 +4,6 @@ Using Yosys (advanced)
 .. toctree:: 
 	:maxdepth: 2
 
+	synthesis/index
 	more_scripting/index
-	memory_mapping
 	yosys_flows

docs/source/using_yosys/more_scripting/index.rst

@@ -4,8 +4,6 @@ More scripting
 .. toctree::
 	:maxdepth: 3
 
-	opt_passes
 	selections
 	interactive_investigation
-	synth
 	troubleshooting

docs/source/using_yosys/synthesis/index.rst

@@ -0,0 +1,10 @@
+Synthesis in detail
+-------------------
+
+.. toctree::
+   :maxdepth: 3
+
+   synth
+   opt_passes
+   proc
+   memory_mapping

docs/source/using_yosys/synthesis/proc.rst

@@ -0,0 +1,4 @@
+The :cmd:ref:`proc` command
+---------------------------
+
+text