doc/hooks/cernlib: document CERNLIB hook (#341691)

# CERNLIB {#cernlib-hook}

This hook sets the `CERN`, `CERN_LEVEL`, and `CERN_ROOT` environment variables. They are part of [CERNLIB's build system](https://cernlib.web.cern.ch/install/install.html), and are are needed for some programs to compile correctly.

autopatchelf.section.md

bmake.section.md

breakpoint.section.md

cernlib.section.md

cmake.section.md

gdk-pixbuf.section.md

ghc.section.md