Keep header section always visible

Move towards empowering the design system identity by having a fixed point of reference on the sidebar. Eventually the sidebar and fixed point of reference could include the Pajamas logo and text instead of the GitLab logo and text. See discussion in https://gitlab.com/gitlab-org/design.gitlab.com/merge_requests/216#note_134508666.

Here's a screenshot from a typical browsing of the design system in action:

Before	After

Fix #188 (closed).