[core] fix(Button): use 12px font in small buttons

[adidahiya]

Fixes #3448

Checklist

• Includes tests
• Update documentation

Changes proposed in this pull request:

• Update small buttons to use 12px font
  • The mixin I changed is only used in button and tag input styles, and it doesn't have any effect on the latter, so it's ok to change the mixin. The font-size declaration matches the same place it's defined for large buttons.
• Add a radio-style button group size selector to the button example in the docs

Reviewers should focus on:

N/A

Screenshot

[blueprint-bot]

[core] fix(Button): use 12px font in small buttons

Previews: documentation | landing | table