manual: Sort overview listing by manual order.

In the glibc manual we have a "Roadmap to the manual" section at
the end of the "Introduction" chapter.

The introductory text says "Here is an overview of the contents
of the remaining chapters of this manual.", but then proceeds to
list chapters out of order and some chapter are never referenced.

This commit reorders the overview to correctly match the manual
order.

See:
https://sourceware.org/ml/libc-alpha/2014-02/msg00823.html