Document suggested platform names

[m-mohr]

The documentation for storage:platform links to a chapter about the platforms, but the values given there don't correspond to the values given in examples. What platform names should be used for common/popular platforms? This would improve interoperability and clients could link to more useful information if a common set of values would have been agreed on.

[duckontheweb]

I ran into this as I was implementing this extension in PySTAC for stac-utils/pystac#734. The JSON schema uses an enum but it's not clear from the README that you have to use one of these values.