Rename manual.scrbl to simply-scheme-manual.scrbl

[soegaard]

The Racket documentation server can't handle that multiple packages use "manual.scrbl" for the manual name.
The result is that the documentation for Simply Scheme doesn't show up on docs.racket-lang.org .

This PR simply renames the file to "simply-scheme-manual.scrbl" in order to avoid the conflict.

[sorawee]

You need to modify info.rkt as well.

But also, you can use info.rkt to control the doc name without renaming the doc file, which e.g. might be better because it preserves git history. Renaming it might have other pros though, so feel free to do whatever works best.