doc: convert git-send-email synopsis and options to new style

doc: convert git-send-email synopsis and options to new style

Replace [verse] with [synopsis] in the SYNOPSIS block and remove
single-quote formatting from the command name.

Backtick-quote all option terms in the OPTIONS section, convert
the standalone placeholder _<encoding>_ in prose, fix single-quoted
server names to backtick form, and convert mbox(5) to `mbox`(5).

Also update config/sendemail.adoc: backtick-quote all configuration
key terms and fix a bare sendemail.aliasesFile reference in prose.

Signed-off-by: Jean-Noël Avila <redacted>
Signed-off-by: Junio C Hamano <redacted>