Add language how to combine (standalone) collections with extensions

[m-mohr]

For implementors that are using STAC to just describe collections and (often) no items, e.g. GEE and openEO, there is currently not enough language in the spec where fields such as those from the eo extension are placed. These extensions are scoped for items, but nevertheless they are useful for collections, too. There is the Commons Extension, but I think it is really difficult to understand for novice users that you can extend the collections with the commons extension to use the eo extension. I think there should be a better/more prominent description about this in the STAC Collection specification.

[m-mohr]

Added some sentences to #422. This describes how GEE and openEO use collections at the moment and is nothing new in that sense. This is probably a temporary solution until we discuss property summaries (#413) and item schemas for 0.8.0.

[m-mohr]

Done.