feat: add buildmanual target, faker demo, subprocess examples, and contributing sidebar to manual

Add Makefile buildmanual target for manual generation and include faker_demo.wren, subprocess_async_demo.wren, subprocess_concurrent_demo.wren, and subprocess_fiber_demo.wren example scripts. Update manual sidebar across api pages to link new web-server tutorial and contributing section with module architecture, pure-wren/c-backed modules, foreign classes, async patterns, testing, and documentation pages.

Makefile

@@ -1,6 +1,6 @@
 # retoor <retoor@molodetz.nl>
 
-.PHONY: build tests clean debug
+.PHONY: build tests clean debug sync-sidebar buildmanual
 
 build:
 	cd projects/make && $(MAKE) -f wren_cli.make -j $$(nproc 2>/dev/null || sysctl -n hw.ncpu 2>/dev/null || echo 4)
@@ -25,3 +25,6 @@ install:
 
 clean:
 	cd projects/make && $(MAKE) -f wren_cli.make clean
+
+buildmanual:
+	python3 util/build_manual.py