Add 'getting started' extensions documentation (#9536)

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

hello/GEMINI.md

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+# Ink Library Screen Reader Guidance
+
+When building custom components, it's important to keep accessibility in mind. While Ink provides the building blocks, ensuring your components are accessible will make your CLIs usable by a wider audience.
+
+## General Principles
+
+Provide screen reader-friendly output: Use the useIsScreenReaderEnabled hook to detect if a screen reader is active. You can then render a more descriptive output for screen reader users.
+Leverage ARIA props: For components that have a specific role (e.g., a checkbox or a button), use the aria-role, aria-state, and aria-label props on <Box> and <Text> to provide semantic meaning to screen readers.

hello/gemini-extension.json

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+{
+  "name": "context-example",
+  "version": "1.0.0",
+  "contextFileName": "GEMINI.md"
+}