docs: add a11y docs

[geoffrich]

Add docs around SvelteKit's accessibility features and what developers need to do to play nicely with them.

I focused these docs on SvelteKit-specific concerns (route announcements, focus management) instead of general a11y practices (e.g. skip link, button vs link, etc). In the future, a "Guides" section on learn.svelte.dev may be a good place to go deeper into general a11y best practices, or integrated into the official SvelteKit tutorial.

I'm looking for feedback on prose, accuracy of information, and thoughts on anything else we want to include (or exclude).

Leaving in draft for now until the dust settles on the big routing changes.

Prior art:

• Next.js
• Remix
• Ember

Context around route announcement/focus management: https://github.com/sveltejs/kit/issues/307

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[accuser]

@geoffrich this is a great start, and the snippets you have included are really helpful.

My only caution is that getting a11y right requires a lot more thought, and I wonder if this needs to be more explicit in the documentation? i.e., whilst Svelte/SvelteKit provides some really good helpers, on their own they don't make an a11y app.

You're already thinking about this with the proposed 'Guides' section on learn.svelte.dev, but helping people be aware of the built-in capabilities as well as understand that these alone don't make an accessible app would be a really open approach to building a better AX.

[geoffrich]

My only caution is that getting a11y right requires a lot more thought, and I wonder if this needs to be more explicit in the documentation? i.e., whilst Svelte/SvelteKit provides some really good helpers, on their own they don't make an a11y app.

@accuser would something to this effect in the intro be what you're looking for? "Keep in mind that while SvelteKit aims to provide an accessible foundation, you are still responsible for making sure your application code is accessible. If you're new to accessibility, see the "Further Reading" section of this guide for additional resources."

I had something similar to that in one of my drafts, but it got lost along the way.

[geoffrich]

The big +page PR merged, so this is ready for review now.