Partial fix link references color in high contrast

[compulim]

Fixes #4907.

Changelog Entry

Fixed

• Fixes #4907. Link references in high contrast mode should use LinkText color, by @compulim, in PR #4908

Description

Link references of non-URL citations should use LinkText for color.

For "external link" icon in Markdown, they are shown in black and we have a bug #4906 to track the issue.

Specific Changes

• Updated StyleSet/LinkDefinitions.ts for high contrast
• Added "run options" for axe-core/shift-left to disable color-contrast rule for tests in high contrast
  • The placeholder (grayed out) texts are in green and they are not acceptable on color contrast. However, this is OS default

• I have added tests and executed them locally
• I have updated CHANGELOG.md
• I have updated documentation

Review Checklist

This section is for contributors to review your work.

• Accessibility reviewed (tab order, content readability, alt text, color contrast)
• Browser and platform compatibilities reviewed
• CSS styles reviewed (minimal rules, no z-index)
• Documents reviewed (docs, samples, live demo)
• Internationalization reviewed (strings, unit formatting)
• package.json and package-lock.json reviewed
• Security reviewed (no data URIs, check for nonce leak)
• Tests reviewed (coverage, legitimacy)