Add disclaimer to rendered book?

The Rust Reference has a big fat red disclaimer at the top of every page:

For now, this reference is a best-effort document. We strive for validity and completeness, but are not yet there. In the future, the docs and lang teams will work together to figure out how best to do this. Until then, this is a best-effort attempt. If you find something wrong or missing, file an issue or send in a pull request.

In comparison, the rendered "RFC book" has nothing like that. That's a problem, because we generally don't update RFCs after they have been accepted -- so they are frequently outdated or at least not explained in the best possible way.

So I suggest we also add a disclaimer to the RFC book, saying something like
"An RFC is a historic document that reflects the consensus at the time it was accepted. They are, in general, not updated when things change later. For current information, check the reference and the nomicon."

Cc @Centril @gnzlbg

Start by locating the source for the rendered RFC book and its page-level introductory content. Add a disclaimer explaining that RFCs are historical and may be outdated, with links to the Rust Reference and Nomicon. Done means the disclaimer appears in the rendered book; no test or specific source file is named.