[Accessibility] Add tooltip to MinimalFooter icons

[simurai]

There was a recent accessibility audit for https://githubuniverse.com.

Expected result
On the footer section, the tooltips should be defined for the twitter, github, linkedin, youtube and facebook controls.

The MinimalFooter component currently has the visually-hidden text for screen readers. But I assume we would also need something for mouse and keyboard users that might not know what the icon (e.g. twitter bird) means. In the audit it says adding a "tooltip", but not sure if it needs to be a Tooltip like we have in Primer React? It's still marked as "Not reviewed for accessibility". Or if adding a title attribute that the browser shows as a tooltip would be enough? Maybe something to get clarity with the #accessibility team first.

[joseph-lozano]

Discussed with a11y working group at the Accessibility Eng+ Office Hour. This is no longer considered a WCAG failure. We will follow up with this when Primer implements its Tooltip component (currently deferred to Q2).