Whitespace guidelines in UX Guide

Would be great to have some guidelines for space between and around elements

Examples:

Basics

• Unit used (px, em, or other)
• Size for small / normal / large spaces
• Using divider lines vs whitespace

Layout

• Vertical rhythm guidelines
• Space between related items
• Space between separate sections

Components

• Alignment of labels with inputs (e.g. margin-right amount for inline labels)
• Input field / button padding
• When to add padding around grouped elements, e.g. list items - see https://gitlab.com/gitlab-org/gitlab-ce/issues/29555#note_25944657

/cc @pedroms