[META] Accessibility of Olivero header/menu region

Adding this placeholder issue (so it has an issue number...).

Discussed with @mherchel, @katannshaw, and the accessibility topic maintainers during the accessibility office hourse meeting (15th Oct 2020).

I'll flesh this out with child issues. I have detailed notes already.

Removing a child issue - #3190113: Hover style on level-2 nav not applied to full width of <a> doesn't have much bearing on accessibility.

Added #3191077: Olivero narrow/mobile menu constrains tabbing in one direction only..

Note that this is much more urgent than #3190242: Nav should use core/drupal.tabbingmanager instead of custom focus manager (which is a task, not a bug).

Note that we (loosely) decided to follow the Link + Disclosure Widget Navigation pattern during the summer last year.

The way this emerged is that we were reviewing the behaviour of the menu at a time when it didn't have the buttons yet. IIRC it was the first time the Olivero team showed the work to the accessibility maintainers, and @mherchel and I continued discussing it on Slack afterwards while he implemented it. After we aligned on the use of a button for each submenu, I noted that it was now becoming VERY similar to the Link + Disclosure Widget Navigation, and proposed that we follow that pattern closely. Meanwhile, @mherchel wrote an article along similar lines; Accessible Navigation with Drupal Core’s Menu System

I don't know if we ever explicitly stated that in the issue queue though. The menu still follows that pattern very closely, so I'm adding it to the issue summary here now. At this stage, I don't think we should depart from that pattern.

First cut at must/should/could triage.

Added #3191806: Only close Olivero sub-menus when resize results in a different menu format

Added #3192903: Mouseout event should not close navigation sub-menu if focus is inside the sub-menu