Generate an AsyncAPI documentation for the Mercure API.

[dunglas]

As well as our OpenAPI component, we should add a new component to expose an AsyncAPI documentation.
Initially, the docs should expose the Mercure topics.

[dmsmidt]

I'm reevaluating API Platform and was wondering if there has been recent talk about adopting AsyncAPI.

[dunglas]

@dmsmidt not AFAIK, but as API Platform already provides all the building blocks to build JSON Schema and OpenAPI docs, adding support for AsyncAPI should be quite easy.

PR very welcome 😁