Fix markdown book links
The redox book contains some broken links throughout the various markdown pages. Fix all broken links (to existing pages) such that the documentation can be used as intended.
Additionally, src/explore/explore.md contains a link to a "Utilities" file. This file no longer exists (it was removed in commit 2854c9b0). In order to avoidunnecessary 404-type errors, remove the link to this page.