docs/website: add page for the Buildroot Association

For about two years, a legal entity called "Buildroot Association" was
created in France to support the Buildroot project. Until fall 2018,
this legal entity has not been used. In fall 2018, we started using it
in order to receive donations from companies in order to organize the
Buildroot Developers Meeting that took place before the Embedded Linux
Conference Europe 2018 in Edinburgh.

This commit creates a new page on our web site that documents the
existence of this Buildroot Association, and details how to become a
member. Both individuals and companies can become members.

It is worth stating that the Buildroot Association does not control
the Buildroot project: it remains a fully open-source and
community-driven project. The Buildroot Association only serves as a
legal entity to handle donations and money needed to organize the
Buildroot Developers Meeting, and pay for a few expenses related to
the project (such as domain names).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
[Arnout: small improvements]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>