Create STAC API best practices

[m-mohr]

Related links:

• Move "best practices" text into separate repo stac-api-spec#391
• https://github.com/ceos-org/stac-collection-and-granule-discovery-best-practices

[m-mohr]

Also move the open issues:

• https://github.com/radiantearth/stac-spec/issues?q=is%3Aissue%20state%3Aopen%20label%3A%22best%20practice%22