Add logos to design.ubuntu.com brand section

[lyubomir-popov]

Summary

We recently removed a link to google drive, as it was generating hundreds of requests for access to a folder that was at the time an internal resource. We now have the final, official puiblicly available logos, so it is time to put those back online.

In the visual design meeting we agreed on the following:
@egezeytun will provide a link to the logos next week
@marcushas will write up accompanying guidelines, and we can add those to the logos page once available

cc @bartaz to plan the work in