Add link to stac-fields (fields.json) in best practices

[m-mohr]

Origin: #985 (comment)

The plan is to give users a "definitive" list of fields that don't make sense to summarize and currently the list lives as fields.json in the stac-fields package (summary: true/false flag). We should add the link once stac-fields has matured a bit more and is stable.

[m-mohr]

Machine-readable normalized list from CDN:

https://cdn.jsdelivr.net/npm/@radiantearth/stac-fields@latest/fields-normalized.json

[m-mohr]

I guess we don't need this in the spec anymore. Just have a look at stac-fields if needed :-)