Accessibility updates

[ravicodelabs]

See issue #29 for details.

Updates made:

• Added an invisible level 1 heading for the main content on the home page (the logo image is serving as the "visible" heading). The invisible level 1 heading helps with keyboard navigation via screen readers (e.g. it shows up in the "Headings" menu).
• Updated pagination buttons in the JEDI Corner section to conform to accessibility contrast guidelines.
• Added section on "Accessibility Tips" to CONTRIBUTING.md.
• Provided feedback for the Quarto team:
  • Starting at the top left image (or starting with the navigation landmark), the screen reader flows from the JEDI Outreach Group image and heading, then all the way right to the search icon, and finally to the individual page navigation links. It would be better if the screen reader flowed to the page navigation links before the search icon, as it is visually. Furthermore, the search icon takes three screen reader moves to get past (starting with entering a "combo box"), whereas one would be ideal. (Link: https://github.com/quarto-dev/quarto-cli/issues/7058)
  • A sidebar also adds a navigation Landmark. I think it would be useful for screen reader users if this shows up with a useful and unique label in keyboard navigation menus, e.g. perhaps by using something like <nav aria-labelledby="sidebar navigation">. (Link: https://github.com/quarto-dev/quarto-cli/issues/7059)
    • Our webinars listing page navigation seems to do this automatically, e.g. if you inspect the pagingation button you will see <nav id="listing-pagination" class="listing-pagination" aria-label="Page Navigation">
    • Our activities page navigation is different and does not automatically add the aria-label, e.g. if you inspect the link to the next subpage you will see only <nav class="page-navigation">.

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