Document and test Button component's "small" size

[joshfarrant]

Summary

Adds the new "small" Button size to docs, Storybook, and adds tests for Button size.

List of notable changes:

• add small Button to docs
• add small button story
• add button size tests

What should reviewers focus on?

• Take a look at Storybook and the docs to preview the changes.
• Check that you're happy with the approach of the test that's been added. There aren't any other instances of it.each() in the codebase, and equally you might not think it's necessary to test all size variations.

Supporting resources (related issues, external links, etc):

• https://github.com/github/primer/issues/3149

Contributor checklist:

• All new and existing CI checks pass
• Tests prove that the feature works and covers both happy and unhappy paths
• Any drop in coverage, breaking changes or regressions have been documented above
• New visual snapshots have been generated / updated for any UI changes
• All developer debugging and non-functional logging has been removed
• Related issues have been referenced in the PR description

Reviewer checklist:

• Check that pull request and proposed changes adhere to our contribution guidelines and code of conduct
• Check that tests prove the feature works and covers both happy and unhappy paths
• Check that there aren't other open Pull Requests for the same update/change

Screenshots:

Please try to provide before and after screenshots or videos

Before	After

[changeset-bot]

🦋 Changeset detected

Latest commit: 1ab06cf

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 6 packages

Name	Type
@primer/brand-primitives	Patch
@primer/react-brand	Patch
@primer/brand-e2e	Patch
@primer/brand-fonts	Patch
@primer/brand-config	Patch
@primer/brand-storybook	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[github-actions]

🔍 Design token changes found

View CSS variable changes

- --brand-control-small-paddingBlock: var(--base-size-12);
+ --brand-control-small-paddingBlock: var(--base-size-8);

[github-actions]

🟢 No visual differences found

Our visual comparison tests did not find any differences in the UI.

[rezrah]

👋 @joshfarrant thanks for opening this PR. Just FYI that you can update your visual snapshots locally through Docker, or directly from the PR by adding the 'update snapshots' label. Latter will commit the updated snapshots to your branch.

[rezrah]

Code changes LGTM 👍, nice one @joshfarrant 🙌

@auareyou would you mind giving this a quick design review / sign-off also please 🙏

[auareyou]

LGTM!

[joshfarrant]

@rezrah Added a new changeset, lmk if you have any suggestions 👍