Replace LiteLLM with vLLM in Documentation

Support for LiteLLM was predicated on supporting Ollama. As we have dropped support for Ollama, we no longer need to support LiteLLM.

We do support vLLM but this is not documented, so it needs to be added.

Definition of Done

• Based on results of testing for each model and inference platform on staging ref, determine whether litellm can be removed from our documentation
  • Remove litellm from documentation
    • https://docs.gitlab.com/ee/administration/self_hosted_models/troubleshooting.html#check-if-gitlab-can-make-a-request-to-the-model
    • https://docs.gitlab.com/ee/administration/self_hosted_models/litellm_proxy_setup.html