doc: add documentation for gen_compile_commands.py

doc: add documentation for gen_compile_commands.py

This documentation briefly explains what is a compilation database,
and how to use the script to generate one.

This is not a portage, as there was no original documentation in the
Linux sources.

Acknowledge the documentation in the script's header and in doc/build
index.

Signed-off-by: Joao Marcos Costa <jmcosta944@gmail.com>
Tested-by: Joao Paulo Goncalves <joao.goncalves@toradex.com>