Add example before the specification

[fulldecent]

Regarding the OpenAPI homepage.

The specification goes directly into grammar and details. But nobody has shown me what the OpenAPI specification looks like. An example text file/device/robot/planet/other thing which complies with the specification should be shown as a specimen to explain what OpenAPI is.

[aghorpade]

@fulldecent - I guess you need to take a look at https://petstore.swagger.io/ to understand what OpenAPI specification looks like once you use them.

[fulldecent]

Proposed fix at #2299

[lornajane]

I think we could signpost the (newer than this issue) resources at https://learn.openapis.org to support this request.

[handrews]

PR for the part of this warranting a spec change is merged for 3.0.4 and ported to 3.1.1 via PR #3921!
As noted above, the rest of this belongs with the Learn repo.