git-imap-send.txt: don't duplicate 'Examples' sections

git-imap-send.txt: don't duplicate 'Examples' sections

Remove the 'Examples' subsection in the 'Configuration' section and move
these examples to the 'Examples' section. Also remove the 'Variables'
title since it is now useless.

Also, use appropriate Asciidoc syntax for configuration values, and
capitalize 'Gmail' properly.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>