bug fix: Package doc html files into deb package

To get the help html files dinamically the Makefile was using the
following code:

DITA_HTML_FILES = $(patsubst %.dita,%.html,$(wildcard *.dita))
$(wildcard *.html)

But it does not well while building deb packages as it cointans
duplicated values:

 /bin/mkdir -p '/tmp/tmp.86vGERYO70/usr/share/kimchi/ui/pages/help'
 /usr/bin/install -c -m 644 guests.html storage.html templates.html
guests.html index.html storage.html templates.html
'/tmp/tmp.86vGERYO70/usr/share/kimchi/ui/pages/help'
/usr/bin/install: will not overwrite just-created
`/tmp/tmp.86vGERYO70/usr/share/kimchi/ui/pages/help/guests.html' with
`guests.html'
(...)

So use $(if()) statement to get html files from one source or from
another to avoid the errors above.

Signed-off-by: Aline Manera <alinefm@br.ibm.com>
doc: Generate index.html to join all help pages

This patch generates help/index.html automatically based on all .dita
files during build time.
For each dita file it generates a link on index.html

Signed-off-by: Aline Manera <alinefm@br.ibm.com>