Rollup merge of #57929 - GuillaumeGomez:rustodc-remove-old-style-files, r=ollie27

Rustdoc remove old style files

Reopening of #56577 (which I can't seem to reopen...).

I made the flag unstable so with this change, what was blocking the PR is now gone I assume.

src/librustdoc/lib.rs

@@ -345,6 +345,11 @@ fn opts() -> Vec<RustcOptGroup> {
                        "Directory to persist doctest executables into",
                        "PATH")
         }),
+        unstable("generate-redirect-pages", |o| {
+            o.optflag("",
+                      "generate-redirect-pages",
+                      "Generate extra pages to support legacy URLs and tool links")
+        }),
     ]
 }