replace collections extension with commons extension

[matthewhanson]

Changes the Collections extension with the Commons extension.

This allows putting properties that belong to an Item into it's Collection and have it apply to all member Items.

[m-mohr]

Not reviewed yet, but I'd like to have all extensions in folders. Therefore I'd suggest moving the file in a commons folder. Same should be done with the EO extension, but that can be a separate PR of course.

[matthewhanson]

@m-mohr I did move it into a 'commons' folder.

[m-mohr]

Thanks for moving the extension to a separate folder. I went through the document and fixed some issues regarding the formatting and also fixed JSON validation errors. Other than that it is great work. I'm generally fine with everything now and would approve, but I am missing two things.

1. A full example in the examples folder.
2. I'd also like to add in a paragraph that if you are only using Collections without linking to Items, that it is fine to use the extension as well. At least in openEO, I'll encourage the implementors to just use the Commons extension and add all their fields from the Common Metadata Model to describe their collections without actually linking to their items as we usually have big data cubes that are not available for download.

[matthewhanson]

@m-mohr examples added and README updated

[hgs-msmith]

I do think this is a better name, and likely to reduce confusion.