Update docs about inference endpoints

[albertvillanova]

Update docs about inference endpoints, once this PR was merged:

• Autoscaling inference endpoints #412

In particular:

• Remove type: "endpoint"
• Rename model to model_name
• Explain to pass model_name or endpoint_name+reuse_existing (this defaults to false)
• Fix old instance type/size that are no longer valid

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.