Move best practices out of stac-spec?

[cholmes]

A few people have pointed out that the best practices document is a bit of a monster. There's lots of good information in there, but it's at varying levels of 'maturity' in terms of how solid the practice is.

We should consider moving it to another repository and just linking to it. We thought about doing this for RC1, but there are too many references to it. May want to consider putting some things like CORS more into the core spec.

If we have time it might make sense to transform it into one or more 'guides'.