[📑 Docs]: Provide content for `AsyncAPI Specification File` document

[derberg]

What Dev Docs changes are you proposing?

https://github.com/asyncapi/generator/blob/master/docs/asyncapi-file.md- explain in short one paragraph what asyncapi specification is, no more, just one sentence and link to the spec. We assume people know, but if someone is lost, just have this sentence and link to docs

• maybe some simple example needs to be show?
• explain that AsyncAPI file can be yaml or json. First paragraph from https://github.com/asyncapi/spec/blob/master/spec/asyncapi.md#specification is good I think
• explain that generator instance takes the AsyncAPI file and before Generator Template gets it as input, it is parsed by the JS Parser to enable template developer to gain access to set of functions that make it easier to extract data from the AsyncAPI file. This could also be supported by a diagram.

cc @pratik2315 @Florence-Njeri

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