Don't produce TOCs for doc markdown files

Currently, we are producing headers for markdown files,
which is generally not what we want. As such, passing this
flag causes them to render normally.

src/bootstrap/doc.rs

@@ -312,6 +312,7 @@ fn invoke_rustdoc(builder: &Builder, compiler: Compiler, target: Interned<String
     cmd.arg("--html-after-content").arg(&footer)
         .arg("--html-before-content").arg(&version_info)
         .arg("--html-in-header").arg(&favicon)
+        .arg("--markdown-no-toc")
         .arg("--markdown-playground-url")
         .arg("https://play.rust-lang.org/")
         .arg("-o").arg(&out)