docs: Add backreference flow diagram (Figure 3) in rewrite/tech

docs: Add backreference flow diagram (Figure 3) in rewrite/tech

Add SVG source and regenerated PNG for mod_rewrite_fig2 showing how
backreferences flow: $N from RewriteRule Pattern (matched first) to
all RewriteCond TestStrings and Substitution; %N from last matched
CondPattern to Substitution. Insert as Figure 3 in tech.html. Add
crosslink from intro.html backreferences section to the new diagram.
Remove obsolete GIF.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1934612 13f79535-47bb-0310-9956-ffa450edef68