docs: bundled example benchmark suite

Examples covering the main usage patterns:

- bench_append_u8: per-iteration heap work, demonstrates B/op tracking
  via the wrapped allocator.
- bench_sha256_64: set_bytes() for throughput in MB/s.
- bench_integer_sum: shows the optimization pitfall with a comment —
  trivial loop bodies need an in-loop b.keep to survive ReleaseFast.
- bench_hash_sizes: parent that delegates to sub-benchmarks via b.run,
  printed as `hash_sizes/sha256_<size>`.
- memset_{16,256,4096}: comptime-parametric benchmark generation via
  std.fmt.comptimePrint + a generic gen_bench factory.

Includes a stand-alone examples/bench/build.zig illustrating the
zbench_build.add_bench_step integration a downstream project would use.

examples/bench/build.zig

@@ -0,0 +1,26 @@
+//! Stand-alone build for the example, illustrating how a downstream project
+//! wires up `zig build bench` using the `zbench_build` helper.
+//!
+//! Note: when building the example from the zbench repo root via
+//! `zig build example`, this file is unused — the root `build.zig` wires
+//! it up directly. This file demonstrates the integration pattern for a
+//! consumer project that lists `zbench` as a dependency.
+
+const std = @import("std");
+const zbench_build = @import("zbench_build");
+
+pub fn build(b: *std.Build) void {
+    const target = b.standardTargetOptions(.{});
+
+    const zbench = b.dependency("zbench", .{
+        .target = target,
+        .optimize = .ReleaseFast,
+    });
+
+    _ = zbench_build.add_bench_step(b, .{
+        .step_name = "bench",
+        .root = b.path("main.zig"),
+        .target = target,
+        .zbench = zbench.module("zbench"),
+    });
+}