Docs: Create docs landing page (#10453)

The main purpose of this landing page is to allow users to view "https://eslint.org/docs", which currently is HTTP 404 on our site.

As a secondary benefit, this would also allow the landing page to be used as the launching point for Algolia's Docsearch crawler.

docs/README.md

@@ -0,0 +1,17 @@
+# Documentation
+
+Welcome to our documentation pages! What would you like to view?
+
+## [User Guide](user-guide)
+
+Intended for end users of ESLint. Contains information about core rules, configuration, command line options, formatters, and integrations,
+as well as guides for migrating from earlier versions of ESLint.
+
+## [Developer Guide](developer-guide)
+
+Intended for contributors to ESLint and people who wish to extend ESLint. Contains information about contributing to ESLint; creating custom
+rules, configurations, plugins, and formatters; and information about our architecture and Node.js API.
+
+## [Maintainer Guide](maintainer-guide)
+
+Intended for maintainers of ESLint.