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!
It's written as 4*a+b inside the parentheses. I wanted to show the first expression the way students would normally think of it, and then remind them (by adding the * in what I tell them to type) that they need to indicate multiplication explicitly in addition to adding the sqrt function.