Auto merge of #4476 - behnam:book, r=alexcrichton
authorbors <bors@rust-lang.org>
Sat, 9 Sep 2017 06:08:08 +0000 (06:08 +0000)
committerbors <bors@rust-lang.org>
Sat, 9 Sep 2017 06:08:08 +0000 (06:08 +0000)
[doc/book] Move section content to */index.md

This gives a better over-all structure to the book, which we can also
follow for other books on <doc.rust-lang.org>.

Also, confirming fix for broken links with the latest (0.0.24) `mdbook`
release.


Trivial merge