Sylvia/docs/sfd

@mhoeber @lamagnifica SFD_ORA.rst and SFD_mathformatting.rst are ready for a dev review when you guys have the time. (SFD_certificates is still in the writing phase, so ignore that one for now.) Thanks!
you describe using explicit multiplication over 150 lines earlier. I think it would be nicer to use an example of only the concept you're describing here, and then only include a more complex example such as this one along with a reminder that they need to use parens for functions...