scripts/doc_to_xls.py: split code into a XLS conversion and main()

Such change allows using the code on some other python script that
would be converting IGT doc test documentation into XLS files.

Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Acked-by: Katarzyna Piecielska <katarzyna.piecielska@intel.com>
Reviewed-by: Kamil Konieczny <kamil.konieczny@linux.intel.com>