Show "PDF are currently being built" instead of a 404

[JulienPalard]

From an idea of @Mariatta, when we generate the HTML only for a new branch, we could generate a static HTML page containing a nice message like:

This PDF file is currently being built, please try again in a few moments.

It would allow to split again fast HTML build and delayed PDF builds without having the old flood of "the PDF are broken!" in docs@ mailing list.