Clarify when to use tabs versus button groups

What does this MR do?

Adds behavioral and semantic guidance to distinguish tabs from button groups, and clarifies when to use and not to use each.

This revives the conversation from !4323 (closed), which was closed before merging. Visual alignment work in !5867 is intentionally bringing tabs and button groups closer together visually, so the behavioral and semantic differentiation in the docs is critical to prevent misuse.

Does this MR meet the acceptance criteria?

• The “What does this MR do?” section in the MR description is filled out, explaining the reasons for and scope of the proposed changes, per “Say why not just what”.
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Links

• Revives !4323 (closed)
• Relates to #770 (closed)
• Relates to !5867