More updates for the URL library manual.

More updates for the URL library manual.

* url.texi (Introduction): Move url-configuration-directory to
Customization node.
(Parsed URIs): Split into its own node.
(URI Encoding): New node.
(Defining New URLs): Remove empty chapter.
(Retrieving URLs): Add an introduction.  Doc fix for url-retrieve.
Improve docs for url-queue-*.
(Supported URL Types): Copyedits.  Delete empty subnodes.