Fix docs site routing and improve GH-pages deployment

[cjcenizal]

We should essentially follow the steps here: https://create-react-app.dev/docs/deployment/#github-pages

Currently, loading a docs deep-link results in a 404, e.g. https://vectara.github.io/vectara-ui/summary. This is because of this issue: https://create-react-app.dev/docs/deployment/#notes-on-client-side-routing. This link proposes solving this issue:

Alternatively, you can use a trick to teach GitHub Pages to handle 404s by redirecting to your index.html page with a custom redirect parameter. You would need to add a 404.html file with the redirection code to the build folder before deploying your project, and you’ll need to add code handling the redirect parameter to index.html. You can find a detailed explanation of this technique in this guide.