Improve documentation around national centers

[MaxMyzer]

Documentation in question: https://www.weather.gov/documentation/services-web-api - under "Specification", "schemas", "Office" type, along with any applicable sub-types. I am opening this here because my past emails did not get a response.

National Centers for Environmental Prediction (NCEP) offices, such as the SPC, WPC, SWPC, HPC, and others do not have their codes listed in the valid list. This also applies to the API response for the listing of locations as either.

Here is a list of many national office IDs that are not listed. Some of these are inactive. https://www.weather.gov/source/datamgmt/xr07_Center_ID_List.html

I also want to note, this would preferably be on the api web service site, not just the github.io site.