Move "Introduction to Supply and Demand"

[jstac]

I propose that we move https://intro.quantecon.org/intro_supply_demand.html to the start of "Essential Tools" and rename it "Supply and Demand for a Single Good".

One reason is that it's currently after https://intro.quantecon.org/linear_equations.html, which discusses supply and demand for multiple goods.

It might be nice to add

• a few sentences at the top on how the single good supply and demand model is one of the model famous models in economics, and an ideal entry point into economic reasoning
• some preliminary discussion of integrals -- how they measure area under curves, and which simple rules for integration we will use

Finally, let's replace $dx$ with $\mathrm{d} x$ so that expressions like

$$ \int_0^q (d_0 - d_1x ) dx $$

are easier to read (i.e., use \mathrm{d} x)

@thomassargent30 , please let me know your thoughts.

[jstac]

I suggest that we replace the class with a NamedTuple and make the methods functions. This will make the lecture more aligned with later lectures.

It's also more consistent --- why are some functions methods of a class and others stand-alone functions that take the class instance as a parameter?

[jstac]

Related to #358

[mmcky]

• the rework and updates has been done in [intro_supply_demand] Adding to supply and demand lecture #372
• suggestion re: toc is in ENH: Adjustments to TOC (two parts for linear dynamics, rework essential tools) #377