Makefile: Combine gen_images and gen_examples

gen_images and gen_examples are never called on their own, CI scripts call make -C docs directly. Since calling them both in parallel seems to cause issues, let's not do that, and instead combine them into a singular `make docs/gen`.  This should resolve the parallelism problems by making them sequential while still retaining the -j support.

Makefile

@@ -996,12 +996,9 @@ docs/source/cell/word_add.rst: $(TARGETS) $(EXTRA_TARGETS)
 docs/source/generated/cells.json: docs/source/generated $(TARGETS) $(EXTRA_TARGETS)
 	$(Q) ./$(PROGRAM_PREFIX)yosys -p 'help -dump-cells-json $@'
 
-PHONY: docs/gen_examples docs/gen_images docs/guidelines docs/usage docs/reqs
-docs/gen_examples: $(TARGETS)
-	$(Q) $(MAKE) -C docs examples
-
-docs/gen_images: $(TARGETS)
-	$(Q) $(MAKE) -C docs images
+PHONY: docs/gen docs/guidelines docs/usage docs/reqs
+docs/gen: $(TARGETS)
+	$(Q) $(MAKE) -C docs gen
 
 DOCS_GUIDELINE_FILES := GettingStarted CodingStyle
 DOCS_GUIDELINE_SOURCE := $(addprefix guidelines/,$(DOCS_GUIDELINE_FILES))
@@ -1037,7 +1034,7 @@ docs/reqs:
 	$(Q) $(MAKE) -C docs reqs
 
 .PHONY: docs/prep
-docs/prep: docs/source/cmd/abc.rst docs/source/generated/cells.json docs/gen_examples docs/gen_images docs/guidelines docs/usage
+docs/prep: docs/source/cmd/abc.rst docs/source/generated/cells.json docs/gen docs/guidelines docs/usage
 
 DOC_TARGET ?= html
 docs: docs/prep

docs/Makefile

@@ -252,6 +252,11 @@ images:
 	$(MAKE) -C source/_images
 	$(MAKE) -C source/_images convert
 
+.PHONY: gen
+gen:
+	$(MAKE) examples
+	$(MAKE) images
+
 .PHONY: reqs
 reqs:
 	$(PYTHON) -m pip install -r source/requirements.txt