Write developer manual

[raxod502]

straight.el is pretty complicated. Even though each part its code is pretty well documented, I'm guessing it would be pretty hard for a new contributor to figure out how the various systems interact with each other and how different formats of information are transformed into each other. There should be some explicit documentation on this.

[ErnestKz]

In my current situation, I'd love for this to exist. I'm tyring to learn how elisp works at a package development level and thought it'd be a good idea to trudge through the source code of this package and use-package since they are the ones that I rely on the most.

[zetaomegagon]

Just mentioning @zetaomegagon (myself) to remember to look at straight.el guts. Maybe I can help at some point.