Accessibility fixes: Homepage

[mariagould]

Accessibility issues on homepage identified in VPAT.

• 1. Add fieldset for ARK radio button on demo

• 2. Remove bold formatting in "Note" after "See How Easy It Is"

• 3. When users switch the identifier type radio button in the demo, focus should not go back to the beginning (currently the page reloads and focus goes back to the beginning)

• 4. Change text in hero image and orange call-to-action button. Hero text should say "Create and manage long-term globally unique IDs for data and other sources". The orange call-to-action button should say "Learn more about EZID"

• 5. Improve contrast in "Create" button in demo

[adambuttrick]

Task 3 addressed with #786.

[adambuttrick]

@sfisher Reviewed these and see two issues:

• Remove bold formatting in "Note" after "See How Easy It Is" - bold tags appear to still be present
• For “Choose an identifier type,” when ARK is selected or when tabbing through the page, DOI is skipped.