perf: Fixup ordering, fix pathing, docs (#38970)

Release Notes:

- N/A

tooling/perf/src/main.rs

@@ -218,7 +218,10 @@ fn compare_profiles(args: &[String]) {
     let ident_new = args.first().expect("FATAL: missing identifier for new run");
     let ident_old = args.get(1).expect("FATAL: missing identifier for old run");
     let wspace_dir = std::env::var("CARGO_MANIFEST_DIR").unwrap();
-    let runs_dir = PathBuf::from(&wspace_dir).join(consts::RUNS_DIR);
+    let runs_dir = PathBuf::from(&wspace_dir)
+        .join("..")
+        .join("..")
+        .join(consts::RUNS_DIR);
 
     // Use the blank outputs initially, so we can merge into these with prefixes.
     let mut outputs_new = Output::blank();