docs: add missing `build` module
docs: add missing `build` module

This was not added because the root `.gitignore` file contained `build/`
(fixed by 4dd70d0 (".gitignore: simplify")).

Signed-off-by: Philippe Proulx <2096628897b40c93960fdd9e24c9c883a54d4fe9@gmail.com>

efficios/barectf,efficios/barectf