docs: change docs rendering and split up markdown files

[ArcaTech]

Closes #222

Notes:

• Split the markdown files into folders for each section (I numbered them since I didn't know how else to dynamically show them in order)
• Added a nav Lume component. I was thinking about making the sections collapsible, but I didn't want to introduce something complicated for state so I just kept all the behavior the way it was. But adding some interactivity in the future will be a little easier with the component.
• The header() function in the docs template doesn't feel good enough. I wanted to do something with a list of urls/titles that can be referenced for the header and nav, but I thought it was better to get this out sooner/simpler and let you decide if you want it improved right now.
• There's some duplication going on with the index.tsx files in each of the docs directories. I tried to make those pages/components as small as possible at least, but it will be nice to figure out a way to clean up the extra files.

[taramk]

@progrium @ArcaTech so for the user guide homepage (https://treehouse.sh/docs/user/), is it possible to put something custom there (like section cards or something) rather than the full user guide text? i can implement it.

[ArcaTech]

Yup. Each of the directories in web/docs (dev, project, quickstart, user) has an index.tsx that displays that section. Right now, each one of those files has the same code that collects every page underneath it and displays them, but you can replace that directly with any HTML you want.