docs/manual: add FAQ entry about speeding up the build process

This is a very frequently asked question, so let's add a FAQ entry
about it, that we can progressively extend with more advices for our
users.

[Thomas: remove the part about your boss buying new hardware, as
suggested by reviewers.]

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>