build.sh can take the information about which contributions to build directly from the main tex file

At the moment, every time there is a new contribution, a change should be applied both to build.sh and to the main tex file. However build.sh can extract the information about how to build from the main tex file, selecting the lines starting with \ia.

The main file for a contribution, if in source form, can be identified by the presence of the \begin{document} string or something similar.