Working on extensions doc

Moved the last files out of the resources directory.
Some tidy up/reformatting of the extensions to allow literalincludes from `my_cmd.cc`.
Most (all?) of the getting started guidelines file is either in the quick guide section, or sections referenced by it.  Instead of including it verbatim, we'll instead just leave a reference to it but then jump straight into the quick guide.
Include an image for the absval generated module.  Still needs more surrounding text but it's good enough for now.

Also includes some other minor tidying, including removing the no longer used abc_01 code example.

docs/source/code_examples/extensions/Makefile

@@ -0,0 +1,29 @@
+PROGRAM_PREFIX :=
+
+YOSYS ?= ../../../../$(PROGRAM_PREFIX)yosys
+
+all: test0.log test1.log test2.log
+
+dots: test1.dot
+
+CXXFLAGS=$(shell $(YOSYS)-config --cxxflags)
+DATDIR=$(shell $(YOSYS)-config --datdir)
+
+my_cmd.so: my_cmd.cc
+	$(YOSYS)-config --exec --cxx $(subst $(DATDIR),../../share,$(CXXFLAGS)) --ldflags -o my_cmd.so -shared my_cmd.cc --ldlibs
+
+test0.log: my_cmd.so
+	$(YOSYS) -Ql test0.log_new -m ./my_cmd.so -p 'my_cmd foo bar' absval_ref.v
+	mv test0.log_new test0.log
+
+test1.log: my_cmd.so
+	$(YOSYS) -Ql test1.log_new -m ./my_cmd.so -p 'clean; test1; dump' absval_ref.v
+	mv test1.log_new test1.log
+
+test1.dot:
+	$(YOSYS) -m ./my_cmd.so -p 'test1; show -format dot -prefix test1'
+
+test2.log: my_cmd.so
+	$(YOSYS) -Ql test2.log_new -m ./my_cmd.so -p 'hierarchy -top test; test2' sigmap_test.v
+	mv test2.log_new test2.log
+