Add Examples for US3, US5 in Datadog Provider Docs

[ryanmaclean]

In the provider docs would i be possible to also include "https://us3.datadoghq.com" and "https://us5.datadoghq.com" as examples? These are the Azure and GCP regions, respectively.

This came up today while going through the docs looking for a "site" variable, and though I'm not sure it needs to be referred to by that name, I do think having these as examples would help.

Further, as the list of examples could get out of date, would having a link to https://docs.datadoghq.com/getting_started/site be relevant?