doc: introduce a synopsis typesetting

doc: introduce a synopsis typesetting

In order to follow the common manpage usage, the synopsis of the
commands needs to be heavily typeset. A first try was performed with
using native markup, but it turned out to make the document source
almost unreadable, difficult to write and prone to mistakes with
unwanted Asciidoc's role attributes.

In order to both simplify the writer's task and obtain a consistant
typesetting in the synopsis, a custom 'synopsis' paragraph type is
created and the processor for backticked text are modified. The
backends of asciidoc and asciidoctor take in charge to correctly add
the required typesetting.

Signed-off-by: Jean-Noël Avila <jn.avila@free.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>