docs: Add [L] flag looping flowchart to rewrite/htaccess.xml

docs: Add [L] flag looping flowchart to rewrite/htaccess.xml

New diagram illustrating how [L] in per-directory context triggers an
internal subrequest that re-enters the ruleset, potentially causing
infinite loops. Shows the [END] flag exit path, the condition-guard
exit path, and the unguarded loop that results in a 500 error.

SVG source and PNG placed in docs/manual/images/. Image referenced
from the "The [L] flag and looping" section of htaccess.xml using the
same figure markup pattern as existing tech.xml diagrams.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1935413 13f79535-47bb-0310-9956-ffa450edef68