Docs examples 02/14: Rework button and button group examples

Part 2 of 14 in the stack that splits up the approved draft !6295 (closed) into reviewable pieces. The content is exactly what was reviewed there; I've only sliced it.

Related to #3575.

Pages: button, button group. This is the pattern exemplar for the whole stack: realistic GitLab content in every live example, one canonical example kept at the top of the page, every other example moved beside the guidance it illustrates, and unique descriptive example titles.

Targets the infrastructure MR's branch and is safe to publish without the MRs after it.