Add "Skip to main content" to content pages

[obulat]

Fixes

Fixes #449 by @obulat

Description

This PR adds the "skip to main content" link to the content pages (About, Sources, etc).

It also updates the way VSkipToContent works. Instead of focusing on the first tabbable element inside the "main content", it focuses on the "main content" itself. This way, when you click "skip to content", the screenreader reads the main content instead of reading the first link.

Testing Instructions

Go to /about, and press Tab on your keyboard. You should see the "Skip to content" button appear at the top. When you click on it, if screen reader is on, you should hear the first heading or the text of the main content.

Checklist

• My pull request has a descriptive title (not a vague title like
  Update index.md).
• My pull request targets the default branch of the repository (main) or
  a parent feature branch.
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added or updated tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no visible
  errors.

Developer Certificate of Origin

Developer Certificate of Origin

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[zackkrida]

Just noting: reviewer assignment based on the CODEOWNERS file didn't work properly 😢 I'll add this to the monorepo tasks: #384

[zackkrida]

LGTM. Exciting to review my first frontend PR in the monorepo.

[github-actions]

Full-stack documentation: Ready

https://WordPress.github.io/openverse/_preview/450

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[openverse-bot]

Based on the high urgency of this PR, the following reviewers are being gently reminded to review this PR:

@dhruvkb
This reminder is being automatically generated due to the urgency configuration.

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[dhruvkb]

Approving, with 1 question

[dhruvkb]

I don't understand why tabindex was set to -1 was set here. I could be wrong but I thought the default is to set it to 0.

[obulat]

When the user clicks on the "Skip to main content" link, instead of focusing on the first tabbable element, we are now focusing on the element itself. It can be a main or a div, which is normally not focusable, or at least not focusable in older browsers.

I just realized that you weren't asking why there is a tabindex at all, but why it's set to -1, @dhruvkb. It's because the element does not need to be focusable when you simply press Tab, it only needs to be focusable by Javascript.

This article suggests to use the -1 tabindex. It also shows that sometimes it makes sense to focus the h1 and not the main element. We might want to do that in the future, if the current implementation does not work well enough.