Link directly to Storybook in design.gitlab.com

From a conversation on Slack:

@dmishunov: Hello, team. [...] I can not find the storybook for gitlab-ui components. Have we changed things somehow?

@Taucher2003: It is now at https://design.gitlab.com/storybook

@mle: That should really be linked somewhere in https://design.gitlab.com/ or https://gitlab.com/gitlab-org/gitlab-services/design.gitlab.com (or the README there)

@Taucher2003: It was linked in a Slack announcement, but yeah, having this somewhere outside of Slack where you would actually look for this link would be good.

@dmishunov: Thanks. That's really not clear how to get to that page. But I will bookmark now

@markrian: There are links to Storybook in https://design.gitlab.com on all embedded stories, though there is no direct link to https://design.gitlab.com/storybook itself. Our plan is to make https://design.gitlab.com more useful than Storybook, so there will be no need to visit it any more. It's a long road to get to that point, though, so perhaps we do need a link to it. I'm thinking a dedicated nav entry for it, with a page that documents some of the above. WDYT?