Move "best practices" text into separate repo

[philvarner]

We have a large amount of text that describes "best practices" for using or implementing STAC API that is not essential to the specification defintion. Examples of this are:

• Hierarchical Catalog Recommenations
• Implementation Recommendations

Moving this text out of the spec will allow us to modify and improve it without needing to release new spec versions.

[philvarner]

dismissing after discussion at 2023-Feb-13 STAC meetup.

[m-mohr]

PSC decision from July 2025 is to move the best practices to https://github.com/radiantearth/stac-best-practices