Improve visibility of STAC specification citation

[fmigneault]

The https://github.com/radiantearth/stac-spec/blob/master/CITATION.cff file should probably be embedded in other locations, such as:

• https://stacspec.org/en/about/stac-spec/
• https://stacspec.org/en/get-involved/
• https://stacspec.org/en/about/

Unless one is aware of this specific file and the repository, it is not obvious for non-developers to figure out this reference.

[m-mohr]

I guess this is a stac-site issue, so moving...