docs: improve send-email documentation

docs: improve send-email documentation

OAuth2.0 is a new authentication method that is being used by many email
providers, including Outlook and Gmail. Recently, the Authen::SASL perl
module has been updated to support OAuth2.0 authentication, thus making
the git-send-email script be able to use this authentication method as
well. So lets improve the documentation to reflect this change.

I also had a hard time finding a reliable OAuth2.0 access token
generator for Outlook and Gmail. So I added a link to the such
generators which I developed myself after seaching through lots of code
and API documentation to make things easier for others.

Signed-off-by: Aditya Garg <gargaditya08@live.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>