Rewrite guide: replace index.xml with structured overview

Rewrite guide: replace index.xml with structured overview

The index page was duplicating content from intro.xml (module
capabilities, complexity warnings). Replace with a concise intro
paragraph and a definition-list TOC that describes how the guide
is organized. Rename the mod_rewrite reference link to
"The Reference Manual."

Add TODO.md tracking the remaining cross-file redundancies to
resolve (path stripping, [L]/[END] looping, FallbackResource,
duplicate warnings, etc.). Will be removed when cleanup is complete.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1934081 13f79535-47bb-0310-9956-ffa450edef68