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.

test/web/future_state.wren

@@ -0,0 +1,13 @@
+// retoor <retoor@molodetz.nl>
+
+import "scheduler" for Scheduler, Future
+
+var future = async { 42 }
+
+System.print(future is Future) // expect: true
+
+var result = await future
+
+System.print(result) // expect: 42
+System.print(future.isDone) // expect: true
+System.print(future.result) // expect: 42