docs/website: mention Bootlin Buildroot training courses

Our documentation page already mentions the open-source and freely
available training materials from Bootlin on Buildroot.

It turns out that we now have online training courses accessible to
public registration, which makes them accessible to a wider
audience. It probably makes sense to mention them alongside the
training materials.

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