doc: add "docs" step to build.zig

build.zig

@@ -165,6 +165,15 @@ pub fn build(b: *std.Build) anyerror!void {
     });
     kernel.pie = true;
 
+    const installDocs = b.addInstallDirectory(.{
+        .source_dir = kernel.getEmittedDocs(),
+        .install_dir = .prefix,
+        .install_subdir = "docs",
+    });
+
+    const docsStep = b.step("docs", "Install documentation");
+    docsStep.dependOn(&installDocs.step);
+
     const kernelStep = try arch.addTargetSpecific(b, kernel);
 
     // TODO QEMU binary override