docs: fix an oversight in Makefile.am, stop gitignoring a tracked file

The former was introduced with Asciidoctor support introduction
(c40e3b0a), the latter was mistaken from beginning (5a53018a), but
it didn't matter since docs/boothd.8.txt had already been tracked
by then -- where it may matter, though, are operations like diff'ing.

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>