Fix website accessibility issues

[peterychang]

The Accessibility team has identified 40 high priority and 19 low priority accessibility issues on the autogen website.

Sev-1 and Sev-2 are considered high priority, 3 and 4 are low priority. Issues were categorized based on my own understanding of the issue/opinion.

Ping me once a few of the issues have been completed so I can have the accessibility team verify the changes comply with all requirements

Due date: Mar 24, 2025

[peterychang]

One major we'll face here is that all of the API reference pages are generated via docstrings, and its unknown how much control we'll have over the behavior of the generated pages.

The remaining pages are generated via markdown which should give us a bit more control, but again, its questionable if we'll have the ability to fine tune certain behaviors enough to meet the required specs (especially things like keyboard controls or screen reader behavior)