Make site index ("/") return something

[lorenyu]

Currently the site index "/" i.e. the "homepage" returns 404, even if the app is up and running. This is unintuitive since the homepage of a site should always return something. Moreover, that assumption is what the infra e2e tests rely on, so the fact that the homepage returns 404 causes infra e2e tests to fail (see this example run in platform-test-flask repo https://github.com/navapbc/platform-test-flask/actions/runs/5324396511/jobs/9643634571)

This ticket is to add some sort of response to the index. To keep it simple it could just be a simple title, description, and link to the "/docs" swagger docs endpoint.