Add makefile target to update HTML files in source tree

Add makefile target to update HTML files in source tree

Also remove the creation of the html/ext sub-directory, which has been
unused since revision r245258.

Backport from mainline
2019-10-08  Jonathan Wakely  <jwakely@redhat.com>

	* doc/Makefile.am (doc-html-docbook-regenerate): New target.
	(${docbook_outdir}/html): Do not create unused 'html/ext' directory.
	* doc/Makefile.in: Regenerate.
	* doc/xml/manual/documentation_hacking.xml: Document new target.
	* doc/html/*: Regenerate.

From-SVN: r277413