DOC: remove very long voc estimate equations from singlediode

[mikofski]

Problem
PR #409 added a very long and redundant set of equations in singlediode that are inappropriate there

Solution
remove it from the docstring in singlediode and move into an appendix of the sphinx source

Alternatives

• just delete it and state that estimated Voc has been shown to be in the 4th quadrant.
• move it to singlediode.estimate_voc
• abbreviate it so it's not so long

Additional context

[cwhanse]

Concur with recommended solution, I don't like the alternatives (move it elsewhere in the code, or delete it). This kind of stuff belongs in a manual.