Add GEDI collection data dictionaries to docs

[chuckwondo]

Users often have trouble determining exactly which datasets are available in the granule files for the various GEDI collections we support, so we would like to incorporate the data dictionaries for the collections within the docs in our repo. The data dictionaries are hard to find and they can also take a moment to decipher, so we'd like to not only incorporate their information in our docs, but also make the information a bit easier to grok.

All of the data dictionaries are available via the corresponding DOI links provided in our MAAP_USAGE.md file, but the links on the DOI pages are not obvious. Further, MAAP_USAGE.md should be moved out of the src/gedi_subet directory and into (perhaps) a docs folder.