Add tabbing behavior

[kaylakremer]

Requirements

• Filling out the template is required. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
• All new code requires tests to ensure against regressions

Description of the Change

I added tab indexes throughout the website so that users can navigate with the use of the tab and shift-tab buttons. In addition to the navigation and links having focus, I also added focus to the paragraphs and list items so that someone with a screen reader will be able to hear the information. Some HTML tags were also edited to be picked up better by a screen reader. For example, a subheader with just a <strong> tag was changed to use <h4> instead.

Benefits

With the tab indexes added, users who rely on a11y should now be able to click on all links and be able to hear all pertinent information on a screenreader.

Applicable Issues

I broke down the tab indexes to focus on every paragraph and list item as mentioned above, but perhaps you might not want this many tab indexes? Also, although I did not add it since it was not asked in the requirements, but it is often a good idea to include a button to skip to main content that is only visible to a screen reader and is focusable. This way, a user does not have to always tab through the nav bar at the top every time they load a page on your site to get to the page's content.

[wordshaker]

From a quick glance, this looks great. I'll give it a thorough test over the next couple of days.