Provide example/documentation on how to implement logout using oauth2-proxy

[jfaltermeier]

Is your feature request related to a problem? Please describe.

Currently we have no example on how to logout/clear the oauth2 session cookie
On the try now page we log out of the auth provider, but this does not clear the session cookies.

Describe the solution you'd like

Maybe a theia menu entry in one of the demo applications which triggers a logout would be nice.

Describe alternatives you've considered

We could also just provide documentation without an example.

Cluster provider

No response

Additional information

See https://oauth2-proxy.github.io/oauth2-proxy/docs/features/endpoints/#sign-out

[jfaltermeier]

See #262