docs: add a page to thank our sponsors

Offer a little bit more visibility to the companies who sponsored us,
either by sponsoring the developer days, or development
boards. Hopefully this will encourage other companies to do the same :)

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