[BOOKINGSG-9357] Fix loading animation for icon-only Button

[qroll]

Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing apis or functionality to change)

Description of changes

• Ensure that the spinner can take up its required width by setting a flex-shrink: 0
• Merged the documentation for the Button variants and reworked the examples to focus on the icon/label and style variants

Checklist

• Changes follow the project guidelines in CONTRIBUTING.md and CONVENTIONS.md
• Looks good on mobile and tablet (the story canvas gets cut off horizontally on mobile - it's happening across multiple components, to revisit another time)
• Updated documentation
• Added/updated tests

Screenshots

Before:

Screen.Recording.2026-05-28.at.11.23.33.AM.mov

After:

Screen.Recording.2026-05-28.at.12.35.16.PM.mov