[READY FOR REVIEW] Adding the accessible-content-guidance to the Guide for Accessibility

[BrainonSilicon]

Added first draft of the accessible content guidance to support the PPIE and Inclusive Events WIP chapters

Summary

This PR is part of the work package to develop the Inclusive and Accessible Events chapter.

• Please refer to Issue [NEW CHAPTER] - Planning Inclusive and Accessible Events #3307 for more information about this work package.

This PR adds the previously reviewed and accepted content from PR #3383 to the Guide for Accessibility.

List of changes proposed in this PR (pull-request)

• Create a new file in the Guide for Accessibility titled accessible-content-guidance
• Add the content from PR 3383 to the new file
• Have the PR be reviewed
• Once reviewed and the Accessibility Working Group has developed a next action, add the page to the _toc

What should a reviewer concentrate their feedback on?

• Should there be any additional content?
• Everything looks ok?

Acknowledging contributors

• All contributors to this pull request are already named in the table of contributors in the README file.

[aleesteele]

Thanks so much for adding this @BrainonSilicon! I'll make sure to review this this upcoming week – will be adding some outlines of the Accessibility guide to #3426 as well that will interact with this PR. 🚀 We're getting there!