Build a dictionary containing the position of the `cells`.

def read_nb_content(cells, mod_name):
    "Build a dictionary containing the position of the `cells`."
    doc_fns = {}
    for i, cell in enumerate(cells):
        if cell['cell_type'] == 'code':
            for match in SHOW_DOC_RE.findall(cell['source']):
                doc_fns[match] = i
    return doc_fns