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.

example/subprocess_async_demo.wren

@@ -0,0 +1,58 @@
+// retoor <retoor@molodetz.nl>
+
+import "subprocess" for Popen
+
+System.print("=== Concurrent Ping Demo (Parallel Streaming) ===\n")
+
+class PingTask {
+    construct new(host, label) {
+        _host = host
+        _label = label
+        _done = false
+        _proc = Popen.new(["ping", "-c", "3", host])
+    }
+
+    tick() {
+        if (_done) return
+        var chunk = _proc.stdout.read()
+        if (chunk == "") {
+            _done = true
+        } else {
+            for (line in chunk.split("\n")) {
+                if (line.trim() != "") {
+                    System.print("[%(_label)] %(line)")
+                }
+            }
+        }
+    }
+
+    isDone { _done }
+    label { _label }
+    host { _host }
+}
+
+var tasks = [
+    PingTask.new("127.0.0.1", "LOCAL"),
+    PingTask.new("8.8.8.8", "GOOGLE"),
+    PingTask.new("1.1.1.1", "CLOUDFLARE"),
+    PingTask.new("9.9.9.9", "QUAD9"),
+    PingTask.new("208.67.222.222", "OPENDNS")
+]
+
+System.print("Starting 5 concurrent pings...")
+for (t in tasks) System.print("  %(t.label) -> %(t.host)")
+System.print("")
+
+while (true) {
+    var allDone = true
+    for (task in tasks) {
+        if (!task.isDone) {
+            allDone = false
+            task.tick()
+        }
+    }
+    if (allDone) break
+}
+
+System.print("")
+System.print("All 5 pings completed concurrently.")