fix: Build new DITA pages when running "make"

When the user types "make" for the first time, all the .dita files are
converted to .html. But if some new DITA page is added to the directory,
it is not build. The user has to remove all HTML files and run
"make" again.

Update the Makefile so it can build new DITA files if any is added

Signed-off-by: Crístian Viana <vianac@linux.vnet.ibm.com>
Build help index page only once

The help index page (ui/pages/help/index.html) is being built once for
every .dita file available. I.e. if there are 10 .dita files, 10 .html
files are generated and index.html is generated 10 times.

Improve the Makefile target so it generates the help index page only once
whenever one of the .dita file changes. The build Python script has also been
updated so its input and output are more flexible, making it easier to handle
from the Makefile.

Signed-off-by: Crístian Viana <vianac@linux.vnet.ibm.com>