Running 'make' or 'make html' will now convert the links to .md files
into links to .html files so that the documentation can be browsed
locally as HTML files. This will also produce the HTML output under
html/.
Running 'make discourse' will convert the links to .md files into links
to the existing discourse pages. The output MD files will be placed
under discourse/
Signed-off-by: Olivier Gayot <olivier.gayot@canonical.com>