Update equation examples to algebra.parse()

[dannbuckley]

Made examples easier to read by changing some of them to algebra.parse()

[ivanistheone]

I like both the explicit construction of Expressions and Equations (current version), but the new algebra.parse way is also good to show because it's faster to type.

What about leaving the initial examples as they are, but show in the bottom how to obtain the same expressions and equations using the algebra.parse?

[nicolewhite]

I think this is fine. The point of this section is to explain how .solveFor() works in different contexts, so I like using the parser here because it's less distracting.