site a11y: make permalinks accessible

[geoffrich]

Resolves multiple Axe accessibility violations (#5678) on the Svelte docs related to permalinks:

• ARIA hidden element must not contain focusable elements (91 count). Permalinks were marked aria-hidden but were still focusable, resulting in a confusing experience for assistive tech users.
• ARIA attributes must conform to valid values (5 count). This is on a few permalinks that have aria-hidden="" instead of aria-hidden="true".
• Links must have discernible text (5 count). Permalinks did not have any text.

To fix this, I removed aria-hidden from the permalinks and added visually hidden text to indicate the purpose of the link. These changes are in a new Permalink component within site-kit and requires that change to be merged first (sveltejs/site-kit#35).

[benmccann]

lgtm. I marked this as a draft for now, but we can switch back as ready to review when there's a version of sitekit with sveltejs/site-kit#35 merged

update: that PR was closed in favor of a PR in the sites repo, which means this PR should probably be against the sites repo as well once we move svelte.dev there. I'll leave this open for now as a reminder

[benmccann]

@geoffrich svelte.dev has finally been migrate to the sites repo, so you should be able to send your PR there now: https://github.com/sveltejs/sites/tree/master/sites/svelte.dev

thanks for the wait. I'll go ahead and close this, but feel free to send a PR against the sites repo