Clarify spacing docs with more explanation and examples
Currently, in Pajamas > Standard spacing guidelines there are no visual examples for spacing and the descriptions of how to apply each size to content are vague.
For example, in this discussion gitlab-org/gitlab#218591 (comment 482797196) it was unclear how the spacing should be applied between groups of inputs and individual inputs.
- Add more detailed explanations for each type of spacing.
- Add examples for each type of spacing.
- Add visuals to support each spacing.