Docs: Add accessbility specific page

Adds an additional page to the developer handbook on accessibility
in particular the landmark regions.

Fixes #3217

Author: mkaz

docs/designers-developers/developers/accessibility.md

@@ -0,0 +1,15 @@
+# Accessibility
+
+Accessibility documentation for developers working on the Gutenberg Project.
+
+For more information on accessibility and WordPress see the [Make WordPress Accessibility Handbook](https://make.wordpress.org/accessibility/handbook/) and the [Accessibility Team section](https://make.wordpress.org/accessibility/).
+
+## Landmark Regions
+
+It is a best practice to include ALL content on the page in landmarks, so that screen reader users who rely on them to navigate from section to section do not lose track of content.
+
+Read more regarding landmark design from W3C:
+
+- [General Principles of Landmark Design](https://www.w3.org/TR/wai-aria-practices-1.1/#general-principles-of-landmark-design)
+- [ARIA Landmarks Example](https://www.w3.org/TR/wai-aria-practices/examples/landmarks/)
+- [HTML5 elements that by default define ARIA landmarks](https://www.w3.org/TR/wai-aria-practices/examples/landmarks/HTML5.html)

docs/manifest.json

@@ -101,6 +101,12 @@
 		"markdown_source": "https://raw.githubusercontent.com/WordPress/gutenberg/master/docs/designers-developers/developers/internationalization.md",
 		"parent": "developers"
 	},
+	{
+		"title": "Accessibility",
+		"slug": "accessibility",
+		"markdown_source": "https://raw.githubusercontent.com/WordPress/gutenberg/master/docs/designers-developers/developers/accessibility.md",
+		"parent": "developers"
+	},
 	{
 		"title": "Data Module Reference",
 		"slug": "data",

docs/toc.json

@@ -18,6 +18,7 @@
 				{"docs/designers-developers/developers/filters/autocomplete-filters.md": []}
 			]},
 			{"docs/designers-developers/developers/internationalization.md": []},
+			{"docs/designers-developers/developers/accessibility.md": []},
 			{"docs/designers-developers/developers/data/README.md": "{{data}}"},
 			{"docs/designers-developers/developers/packages.md": "{{packages}}"},
 			{"packages/components/README.md": "{{components}}"},