Update docs for OpenRouter to use new endpoint

[danbarr]

stacklok/codegate#921 added an endpoint for OpenRouter. Need to:

• Update existing OpenRouter examples on the configuring and Continue pages to use the new endpoint instead of /vllm
• Test with other integrations, add OpenRouter as a first-class provider if it works

[jhrozek]

feel free to use https://docs.google.com/document/d/1URIQNsS-eMork4ABI0RZQh0eeiFo0D3TRZTASpsqIkA/edit?tab=t.0 as bootstrap