Update button ellipsis guidance with decision framework

Summary

Updates the button ellipsis guidance with a clear decision framework based on the extensive discussion and user research in issue #2010 (closed).

Changes

• Replaces vague "additional input" guidance with specific criteria
• Adds decision framework: use ellipsis when action requires input or it's not obvious from context
• Includes practical examples
• Keeps the section concise and actionable without overwhelming detail

Context

The current ellipsis guidance has been unclear and rarely used across GitLab (only 1 implementation found in 3 years). The discussion in #2010 (closed) identified that the guidance needed a decision framework rather than removal, with @chrismicek confirming to "keep the ellipsis guidance, but update Pajamas to include a decision framework."

This simplified version of @pedroms's proposed framework provides clear yes/no criteria while avoiding the complexity of extensive examples or lengthy explanations.

Closes #2010 (closed)