Add default landing page for all integrations

[dogeared]

Right now, we have a default landing page for spring boot apps that use the thyemleaf starter.

This is in service of having an empty application, such as those generated by start.spring.io

We should have a default landing page for all the integrations that has some information as well as a logout post form.

This is so that anyone creating an empty project for any of our integrations can take advantage of all the built-ins we have, including logout, which requires a POST.