Document the "anonymous" Deploy to Netlify & claim later flow

[DavidWells]

Todo update https://docs.google.com/document/d/14hIisP4dSgTAqhcjD_Khj5PWKpni4jEsq4ussqkwoHA/edit# and add to repo with an example.

Note: this is effectively the "reverse" of the normal add-on flow and is for specific user cases

[swyxio]

i'm interested in doing this programmatically i.e. without netlifyctl in the command line. would help with https://github.com/netlify/product/issues/147