Issue-43: Add Skip Link

[juliobranha]

Summary

Fixes #43

This pull request introduces a "Skip to Content" accessibility feature to enhance keyboard and screen reader navigation:

• Adds an accessible "Skip to Content" link at the very top of the page, allowing users to quickly jump to the main content area.
• Implements new SCSS partials for accessibility (_a11y.scss) and layout (_layout.scss) in the assets/scss/ directory.
  • _a11y.scss includes a screen-reader-only mixin to visually hide content while keeping it accessible to assistive technologies, and a skip-link mixin to style skip navigation links for visibility when focused.
  • _layout.scss introduces an auto-margins mixin for horizontally centering elements.
• Adds utility classes:
  • .screen-reader-text (for visually hidden, screen reader-accessible content)
  • .skip-link (for styling the skip link)
• The skip link is hidden visually but becomes visible when focused, ensuring accessibility for keyboard users.

Testing Instructions

• Navigate to the site.
• Use the keyboard (Tab key) to focus on the "Skip to Content" link.
• Verify that the link is visible when focused and allows skipping to the main content.
• Ensure the link is accessible to screen readers.

Checklist

• I have tested these changes locally.
• I have updated documentation as needed.
• I have added necessary unit/integration tests.
• The code style follows the project's guidelines.

[kaitlinbolling]

@juliobranha it looks like this is no longer needed, as WP automatically inserts a skip link on block themes that include a <main> tag. I think we should prefer core's implementation rather than introducing our own. I know I originally filed this issue -- I'm sorry for not noticing this sooner!

See: https://developer.wordpress.org/reference/functions/wp_enqueue_block_template_skip_link/