Convert manual and NEWS from grutatxt to markdown.

Grutatext still works but the project seems to be dead, so we have
to switch to something else eventually.

This patch converts the two grutatext documents in the tree to the
markdown language, getting rid of the dependency on grutatext at
the expense of a new dependency on the markdown executable. Only the
discount markdown converter has been tested and is known to work.

The commit also changes the "Tools" section of the manual to list
discount instead of grutatext.