This is a Homebrew tap with a few SGML-related packages. These are mainly intended for building the PostgreSQL documentation. Other than that, they are ancient, and you probably don't want them unless you know what you are doing.
Just brew tap petere/sgml and then brew install <formula>.
For the PostgreSQL documentation up to version 9.6, you need:
brew install docbook docbook-dsssl docbook-sgml docbook-xsl openjade
export SGML_CATALOG_FILES=/usr/local/etc/sgml/catalog
For PostgreSQL version 10, you only need:
brew install docbook docbook-sgml docbook-xsl open-sp
export SGML_CATALOG_FILES=/usr/local/etc/sgml/catalog
For PostgreSQL version 11 and newer, you don't need this at all anymore.
