update CQL2 implemenation suggestions

[philvarner]

Related Issue(s): #260

Proposed Changes:

1. Update CQL2 implementation recommendations and links to existing impls
2. Recommend implementing CQL2-JSON before CQL2 Text, as the JSON format is significantly easier to parse and most use cases do POST with a JSON body.

PR Checklist:

• This PR is made against the dev branch (all proposed changes except releases should be against dev, not master).
• This PR has no breaking changes.
• This PR does not make any changes to the core spec in the stac-spec directory (these are included as a subtree and should be updated directly in radiantearth/stac-spec)
• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.

[cholmes]

This seems fine, but could be good to write up the reasoning for the switch of recommending text to recommending JSON. (Just in a comment in the PR, not in the spec itself)