Docs: Add documentation (or command) for running test SAML server

When developing this plugin, we need to be able to run a SAML provider to validate and test the WordPress plugin properly integrates with the authentication provider.

This shell command has been provided by the Altis team as a way to spin up a local identify provider:

```sh
docker run --name=localsaml \
-p 8086:8080 \
-p 8443:8443 \
-e SIMPLESAMLPHP_SP_ENTITY_ID=https:/// \
-e SIMPLESAMLPHP_SP_ASSERTION_CONSUMER_SERVICE=https:///sso/verify \
-e SIMPLESAMLPHP_SP_SINGLE_LOGOUT_SERVICE=https:///sso/logout \
-d --rm kristophjunge/test-saml-idp
```

> [!WARNING]
> This docker container is not build for Apple Silicon. If using Docker Desktop, you need to be using the Apple Virtualization Framework and need Rosetta enabled.
>
>

Notes

- The trailing slash on the entity ID is required