Add US National Archives data source

[anjackson]

https://twitter.com/lljohnston/status/1319681370470440961?s=21

https://github.com/usnationalarchives/digital-preservation/tree/master/Digital_Preservation_Plan_Spreadsheet

[anjackson]

Especially now that it's been move to linked data: https://www.archives.gov/preservation/digital-preservation/linked-data

[anjackson]

Just noting that it shouldn't be too difficult to parse and walk, even if the mapping isn't clear yet:

• parse like...
• walk like...

...but it would be good to write up the process in a bit more detail later, in case it helps others.