Accessibility

[hannah-martinez]

• Add title to every page
• Fix sizing and scroll issues on different screen sizes for index and about pages
• Resolve all accessibility issues flagged by the SiteImprove Accessibility Checker

Please check a variety of screen sizes with responsive mode when reviewing to make sure that there's nothing funny happening.

This issue was scoped to the index and about pages because we don't foresee having many active users with accommodations, but please glance through the other pages to make sure that the css changes did not have adverse effects in other places.

Closes #357

[vrose99]

LGTM -- added a few visible titles to some pages for consistency and an overflow issue, but all changes looked good locally + non-proofreading functionality related pages on mobile, too