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* feat: allow any supported payload on /invocations

* update openAPI

* update doc

* doc: Add metrics documentation and add a 'Reference' section

* doc: Add API reference

* doc: Refactor API reference

* fix: Message API link

* Bad rebase

* Moving the docs.

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Co-authored-by: Nicolas Patry <patry.nicolas@protonmail.com>

update messages api docs and add Hugging Face Inference Endpoints
integrations section/instructions

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Co-authored-by: Philipp Schmid <32632186+philschmid@users.noreply.github.com>

This PR simply updates the messages api docs to address content changes
and make format consistent

This PR adds a new page to the docs that describes the Messages API and
how to use it.

Additionally this page will contain cloud provider specific information
for enabling and using this feature. This PR includes a SageMaker
example/information.