Added skeleton loading paradigm to UX guide

What does this MR do?

Adds skeleton loading paradigm to ux guide

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Paradigms need to be documented

Screenshots (if relevant)

See http://localhost:3000/help/development/ux_guide/components.md

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-design/issues/35