Make the redirect page to /app prettier

When I moved the app to /app so that the documentation could be at /docs, I made a bare-bones redirect page from the root page to /app. But at some point, it would be nice to do some design and put some styles on it to make it look nice with the rest of the app.

Start at the bare-bones redirect page from the root page to /app and compare it with the existing app's visual style. Done means the redirect page has styling that looks consistent with the rest of the app; the issue does not identify a file or test to run.

Written by the indexing model from the issue text.