Docs for personal BinderHub CORS setup

Proposed change

Consolidated documentation for setting up CORS headers on personal BinderHub deployments with helm.

Alternative options

Leave the documentation as it is - and I will just be grateful for an already awesome tool!

Who would use this feature?

Anyone wishing to connect to a personal BinderHub deployment via clients that rely on CORS requests, like Juniper.

(Optional): Suggest a solution

My knowledge of CORS headers, etc. is quite limited. But as far as I have pieced together from the MyBinder deployment config/ other Github issues, the following minimal additions must be added to config.yaml:

binderhub:
  cors: &cors
    allowOrigin: "*"

  jupyterhub:
    custom:
      cors: *cors

And I remember reading a comment somewhere from @minrk that only authenticated requests are supposed to work with cors, so maybe the authentication options should also be added?

If I ever get my own cors setup to work, I would be happy to contribute this section to the documentation myself. Unfortunately, my naive attempts with the above options have failed so far. For reference, I am trying to use juniper on this page to connect to this binder. For debugging, the page console is logging connection events as experienced by juniper, which is where I’m seeing the CORS error. I’m not giving up, but maybe someone has some helpful tips in the meantime.