Accessibility: Landing page is missing some landmarks including banner, main, and nav in footer

[jason-me]

Issue

The Sphinx Community landing page is missing some landmarks that enhance screen reader navigation.

Expected Behavior

• WCAG 2.2 A 1.3.1 Info and Relationships requires that information, structure, and relationships conveyed through presentation can be programmatically determined or are available in text.
• Screen reader users expect to hear structural information that will help them navigate a page including important landmark regions.

Acceptance Criteria

• Review Scott O'Hara's Accessible Landmarks blog post
• Add banner, main, and nav in footer to the homepage
• Ensure these elements are properly structured and nested e.g. https://www.html5accessibility.com/tests/roles-land.html
• Test with a screen reader JAWS, VoiceOver, NVDA, Orca, Talkback, etc. - ensure the appropriate landmark regions are announced (e.g. Use Insert + F7 in NVDA)

Linked to tracking issue: #456