(optional) Character string indicating a web page of the Data Repository of the
Brazilian Soil, with options:
"febr": FEBR main web page,
"dictionary": FEBR data dictionary and vocabulary,
"forum": FEBR public forum at Google Groups,
"github": febr package source code repository on GitHub,
"index": FEBR data set index,
"search": data set search web page,
"package": febr package on CRAN,
"template": FEBR spreadsheet template on Google Sheets,
"units": units and conversion factors used in FEBR.