docs: add an OAuth2.0 credential helper for AOL accounts

docs: add an OAuth2.0 credential helper for AOL accounts

Yahoo and AOL, both advertise that they support app passwords for third-party
applications. But generating app passwords for them is broken and unreliable
for quite some time now. Yahoo already had an OAuth2.0 credential helper
added in the documentation, so I thought it would be a good idea to add one
for AOL accounts as well, which is more reliable and secure.

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