Bugfix: missing baseurl content links + back-to-contents skiplink margins

[klepas]

Description

Applies {{ site.baseurl }} to internal links, and adds a quickfix for the margin-top of the back-to-contents links within each section.

Definition of Done

• [ ] Content/documentation reviewed by Jools or someone in the #content-design team
• [ ] UX reviewed by Gary or someone the Design team
• [x] Code reviewed by one of the core developers
• [ ] Acceptance Testing
  • [x] HTML5 validation (CircleCI)
  • [x] Accessibility testing & WCAG2 compliance
• [x] Stakeholder/PO review