chore: add 'Skip to main content' button to the main page

[ToMESSKa]

Closes: INSTUI-4240

ISUUE: main page does not have a a 'Skip to main content' button to avoid going through repetitive content. This current implementation is based on this recommendation: https://www.a11y-collective.com/blog/skip-to-main-content/

TEST PLAN:

• go through the documentation's main page with a screenreader with Tab
• the first focusable element on the page should be a 'Skip to main content' button
• the button should not be visible when not focused
• when the button is clicked, it should bring the focus to the 'Components everyone can count on' header

[github-actions]

PR Preview Action v1.4.8
🚀 Deployed preview to https://instructure.github.io/instructure-ui/pr-preview/pr-1837/
on branch gh-pages at 2024-12-20 10:38 UTC