Add text on remaining Model SIM equations

Add text to section on Godley and Lavoie (2007) Model SIM to describe
remaining equations.

Explanation of how to build a model to follow as a separate input file.

[ci skip]
Split example into two and begin second section
Add pre-amble to model equations
Add explanation of Markdown code blocks

Minor corrections elsewhere.
Further restructure as documentation grows

Switch from two example sections to three:
 - Model theory
 - Model specification and construction
 - Model simulation
Complete section about model script
Reproduce GL2007 Table 3.4 in documentation
Switch theme
Reproduce Figure 3.2 from GL2007
NetworkX no longer an optional dependency