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/client_batch.wren

@@ -0,0 +1,25 @@
+// retoor <retoor@molodetz.nl>
+
+import "scheduler" for Scheduler, Future
+
+var items = ["a", "b", "c"]
+
+var futures = []
+for (item in items) {
+    futures.add(async { "processed_" + item })
+}
+
+System.print(futures.count) // expect: 3
+System.print(futures[0] is Future) // expect: true
+System.print(futures[1] is Future) // expect: true
+System.print(futures[2] is Future) // expect: true
+
+var results = []
+for (f in futures) {
+    results.add(await f)
+}
+
+System.print(results.count) // expect: 3
+System.print(results[0]) // expect: processed_a
+System.print(results[1]) // expect: processed_b
+System.print(results[2]) // expect: processed_c