Improve Accessibility

With this proposal, my aim is to make Oppia's learner-facing pages more accessible by by learners having any form of disability. My suggested method involves two-fold approach. I'll start by addressing the accessibility issues (a11y) that have already been reported on the site. This will entail addressing difficulties with heading-related issues, focus management, colour contrast, and more. Many of these a11y issues have contributors associated to them. While I resolve the unassigned problems, I will be available to help any contributors who get stuck or require further support. Second, I'll utilise Lighthouse to evaluate every Oppia page that is intended for students, raising their accessibility scores to a perfect 100. Each learner-facing page's Lighthouse test audits will need to be examined in order to make the necessary changes. This will include adding alternative text for images, making sure header tags have the right semantic structure, and making sure aria-roles are contained by the appropriate parent. Also, in order to aid contributors in understanding the Lighthouse audit procedure and troubleshooting any potential problems that may emerge throughout testing, I will be adding guidance on how to debug Lighthouse tests to the Oppia documentation.