FormControl a11y issue remediation

[joshfarrant]

Summary

List of notable changes:

• Improved visuallyHidden label documentation
• Improvements to FormControl.Hint
  • Associated FormControl.Hint with the form field with aria-describedby
  • Removed examples where FormControl.Hint was nested inside FormControl.Label
  • Updated FormControl.Hint documentation to clarify that it should only be used as a direct child of FormControl
  • Improved the styling of FormControl.Hint, particularly when used alongside a Checkbox

What should reviewers focus on?

• Check the docs pages that I've modified, including the examples. Make sure they look as they should.
• Check that the FormControl.Hint is correctly associated with the form fields and is accessible.
• Check the snapshots
• Check the stories I've modified look as they should.

Steps to test:

• Link to Docs (todo)

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

• Closes https://github.com/github/primer/issues/3782
• Closes https://github.com/github/primer/issues/3781

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:

Feature	Before	After
FormControl.Hint associated with form field		

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: df03fb2df02c6c591adc0ac78cdd10427a66a6f3

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-e2e | Minor | | @primer/react-brand | Minor | | @primer/brand-primitives | Minor | | @primer/brand-fonts | Minor | | @primer/brand-config | Minor | | @primer/brand-storybook | Minor |

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[github-actions[bot]]

🟢 No design token changes found

[github-actions[bot]]

⚠️ Visual differences found

Our visual comparison tests found UI differences.

Please review the differences by using the test artifacts to ensure that the changes were intentional. Artifacts can be downloaded and reviewed locally. Download links are available at the bottom of the workflow summary screen. ##### Example:  If the changes are expected, please run `npm run test:visual:update-snapshots` to replace the previous fixtures.

Review visual differences