user invites: Small accessibility improvements for homepage invites

Created by: philipp-spiess

Follow up for #32006

This PR implements the suggested changes from @limitedmage in https://github.com/sourcegraph/sourcegraph/pull/32006. Thanks for the feedback!

• The button to toggle the info view is now a proper <Button>
• I created a ticket so we don't forget to change the documentation link (#32253 (closed))
• The × is now behind an aria-hidden (the button already has an aria-label)
• Removed a leftover console.log

Test plan

The biggest changes are the switch from Link to Button so I made sure there are no visual regression:

We also have UI screenshot tests.