docs: Add simplified mod_rewrite overview diagram to rewrite/intro.xml

docs: Add simplified mod_rewrite overview diagram to rewrite/intro.xml

New flowchart at the end of the Introduction section showing the basic
request processing loop: RewriteEngine check, iterate rules, pattern
match, RewriteCond evaluation, substitution, and L/END flag termination.
Explicitly labeled as a simplified overview with a link to tech.xml for
the full processing model. Supplements existing prose for visual
learners without replacing accessible text content.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1935427 13f79535-47bb-0310-9956-ffa450edef68