Rollup merge of #48680 - steveklabnik:no-toc, r=nikomatsakis

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.

https://doc.rust-lang.org/nightly/book/ is an example page currently where this is done incorrectly.

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)