Rollup merge of #26977 - brson:stddoc, r=Gankro

Yet another attempt to make the prose on the std crate page
clearer and more informative.

This does a lot of things: tightens up the opening, adds useful links
(including a link to the search bar), offers guidance on how to use
the docs, and expands the prelude docs as a useful newbie entrypoint.

r? @steveklabnik cc @aturon

src/librustdoc/html/static/main.js

@@ -951,3 +951,8 @@
     }());
 
 }());
+
+// Sets the focus on the search bar at the top of the page
+function focusSearchBar() {
+    document.getElementsByName('search')[0].focus();
+}