docs: redirect to docs.unstructured.io on github pages

[MthwRobinson]

Summary

Updates GitHub pages to redirect to the new https://docs.unstructured.io page. This will appear on GitHub pages after the next tag.

Testing

1. From the docs direction, run make html. You should not see any errors or warnings
2. Open unstructured/docs/build/html/index.html. It should look like the following:

3. Open `unstructured/docs/build/html/404.html`. It should redirect back to `index.html`. Per the [GitHub pages docs](https://docs.github.com/en/pages/getting-started-with-github-pages/creating-a-custom-404-page-for-your-github-pages-site), that page will get served for 404 errors, meaning any links to old docs pages will redirect to `index.html`, which points users to the new docs page.

[cmscmadd]

Thanks Matt!