Update CORS Plugin Documentation for Global Plugin

### Where is the problem?

https://docs.konghq.com/hub/kong-inc/cors/

### What happened?

The global definition is not correct, specifically:

- If the upstream service does not have OPTIONS defined as a method then a 404 error is always returned (this seems like a bug in the implementation also)

See also:

- https://github.com/Kong/kong/issues/4859 which references the OPTIONS
- https://support.konghq.com/support/s/article/404-not-found-of-CORS-OPTIONS-requests - this is incomplete, the example doesn't work as written - needs to specify exactly what the kubernetes definition is

### What did you expect to happen?

Needs text on how this needs to actually find a route, which implies a global OPTIONS route

Needs an example of the default route - see second link

### Code of Conduct and Community Expectations

- [X] I agree to follow this project's Code of Conduct
- [X] I agree to abide by the Community Expectations

Start by checking the replacement documentation repository, then read the CORS plugin page, Kong issue 4859, and the linked support article. Update the documentation to accurately explain global OPTIONS route behavior and include a working Kubernetes default-route example.

Written by the indexing model from the issue text.