docs(a11y): document usage of ARIA landmarks across our products

[giamir]

STACKS-614

This PR documents the usage of ARIA landmarks in our accessibility page.

Furthermore it adjusts landmarks issues across our docs.

How to test

• Download the Landmarks Browser Extension
• Navigate to any page
• Observe we now have 6 landmarks (banner, including global navigation and search - main navigation - main, including table of contents navigation). Screen readers and keyboard-only users can easily jump between landmark sections.

Sidenote: This PR is also adding the aria-current="page" to the selected item in the global and main navigations.

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[giamir]

Overall looks good to me. I noticed the Main navigation does appear twice though (because it has a duplicated as 'guessed') — not sure if that's supposed to happen or not.

The plugin has this feature where it tries to guess if an area is a landmark and it does not really work well. It should be turned off for us developers/designers I guess. You can do that by clicking the preferences section of the plugin