Add Tom's foundational lecture on supply and demand

[jstac]

Shifted from in-work to lectures, added to TOC, corrected spelling and minor typos

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[jstac]

Text to-do

• heading levels need to be consistent (see build error)
• full spell check
• headings need to be capitalized (to fit with other lectures)
• search for ^T and replace with ^\top

Code to-do

• spaces around * and @ (closer to PEP8)
• camel case for classes, no underscore (consistent with PEP8)

[jstac]

@mmcky , I need to work on some of the maths in this lecture tomorrow. Could you or @Smit-create please get it to the stage where it builds in JB, which will make it easier for me to build and edit?

[Smit-create]

Thanks, @jstac. I'm looking into the build failure issue.

[mmcky]

Thanks, @jstac. I'm looking into the build failure issue.

@Smit-create you may want to merge in the latest main re build failures.

[Smit-create]

@jstac @mmcky Thanks. The CI is green again. John, I made a few header changes just to check the CI and make it pass. Please feel to reorder them as you need in the lecture flow.

[jstac]

Thanks @Smit-create !

[jstac]

This PR adds @thomassargent30 's fundamental supply and demand lecture, which has now been broken into three parts

• intro_supply_demand.md (scalar case)
• supply_demand_multiple_goods.md (shifting to multple goods)
• supply_demand_heterogeneity.md (adding heterogeneity)

I'm going to merge this into main and then open separate issues for each lecture to get them ready for publishing.