Authorization Server metadata defined inside implementation considerations

[tplooker]

Caught by @jogu, I think we should move this out into its own top level section.

[jogu]

I might deserve an entry in the acknowledgments too :-)

[babisRoutis]

I also agree that Authorization Server Metadata extensions deserve to be a top-level section
There are the entries defined in implementation considerations and the challenge endpoint.

Somehow related #170