Makefile: explicitly set "manpage" doctype

Although this is already implied for the manpage output, this improves
AsciiDoc's formatting of the HTML output for the manual page.

Signed-off-by: John Keeping <john@keeping.me.uk>
Makefile: allow overriding of "a2x" binary

Signed-off-by: John Keeping <john@keeping.me.uk>
Makefile: generate HTML5 output using "asciidoc" not "a2x"

Using "asciidoc ..." instead of "a2x ..." gives more control over the
HTML output format, since we do not need to use a DocBook toolchain to
generate our HTML output.

Signed-off-by: John Keeping <john@keeping.me.uk>