This effectively reverts 0e63843195 and
173319e7e1.
asciidoc has been revived (for a while now) and doesn't require Python 2. We
still prefer asciidoctor and fallback to asciidoc/a2x if it's not available.
Comparing the asciidoc and asciidoctor man pages, everything looks OK.
Python tends to be available more readily in distribution build environments
rather than the Ruby stack. Also, the pregenerated man pages are gone as of
f132c94c65.
Signed-off-by: Sam James <sam@gentoo.org>