docs: provide md files for openapi site

openapi/instui.md

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+## What is Instructure UI?
+
+Instructure UI (or **InstUI**) is the React component library and design system that powers Instructure's products, including Canvas LMS. It contains 100+ packages (one per component, plus theme, icon, and utility packages) and is published under the `@instructure/*` scope on npm. For most consumers, the easiest way to get started is to add the [`@instructure/ui`](https://instructure.design/#usage) meta-package, wrap your app in the settings provider, and start rendering components. There are no global stylesheets to import and no CSS classes to memorize: styling is handled internally with [Emotion](https://emotion.sh/), and every component is themeable through the same provider.
+
+We aim to make InstUI easy to use, easy to learn, and free from bloat. For this reason every component is built on a few shared principles. All components target [WCAG 2.1 AA](https://instructure.design/#accessibility) (and AAA when paired with the [canvas-high-contrast](https://instructure.design/#canvas-high-contrast) theme), support RTL languages, work with VoiceOver, NVDA and JAWS, and never hardcode user-facing strings, so every label is localizable.
+
+Out of the box InstUI ships with 4 themes. Theming is scoped: you can swap themes for an entire app, or for a subtree, by nesting another settings provider. For teams using AI coding agents, the whole component library is also available as a single downloadable archive of markdown at [instructure.design/markdowns/documentation.zip](https://instructure.design/markdowns/documentation.zip), with an `llms.txt` catalog at [instructure.design/llms.txt](https://instructure.design/llms.txt). For everyone else, the rendered docs at [instructure.design](https://instructure.design) are the best place to start.

openapi/toc.md

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+- [Instructure UI](instui.md)