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/synthesis/synth.rst

@@ -0,0 +1,40 @@
+Introduction to synthesis
+-------------------------
+
+The generic ``synth``
+~~~~~~~~~~~~~~~~~~~~~
+
+The following commands are executed by the :cmd:ref:`synth` command:
+
+.. literalinclude:: /cmd/synth.rst
+   :start-at: begin:
+   :end-before: .. raw:: latex
+   :dedent:
+
+Packaged ``synth_*`` commands
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. todo:: are all these synth commands supported?
+
+The following is a list of all synth commands included in Yosys for different
+platforms.  Each command runs a script of sub commands specific to the platform
+being targeted.
+
+- :doc:`/cmd/synth_achronix`
+- :doc:`/cmd/synth_anlogic`
+- :doc:`/cmd/synth_coolrunner2`
+- :doc:`/cmd/synth_easic`
+- :doc:`/cmd/synth_ecp5`
+- :doc:`/cmd/synth_efinix`
+- :doc:`/cmd/synth_fabulous`
+- :doc:`/cmd/synth_gatemate`
+- :doc:`/cmd/synth_gowin`
+- :doc:`/cmd/synth_greenpak4`
+- :doc:`/cmd/synth_ice40`
+- :doc:`/cmd/synth_intel`
+- :doc:`/cmd/synth_intel_alm`
+- :doc:`/cmd/synth_lattice`
+- :doc:`/cmd/synth_nexus`
+- :doc:`/cmd/synth_quicklogic`
+- :doc:`/cmd/synth_sf2`
+- :doc:`/cmd/synth_xilinx`