Added missing doc strings for all geoms + others

[adknudson]

• Added doc strings for all geoms with a similar format
• removed outdated references to @ggplot and @geom_*
• enabled the use of md strings in literate docs (see https://fredrikekre.github.io/Literate.jl/v2/fileformat/#Syntax)
• updated the contributor's guide for adding examples and building docs locally

Many of the docs for geoms are still missing info about required aesthics, optional aesthetics, and supported options. I am planning to add those in the next few days as I become more familiar with the code base. I'm hoping this PR can be left open until I finish updating all the doc strings.

One other thing that I noticed is that previously the doc string contents were all indented 4 spaces which treated all the contents as being within a code block. I'm not sure if that was intentional, but I have removed the indent.

[rdboyes]

Thanks @adknudson! This all looks great to me - this is my first Julia package so I haven't really gotten used to wrestling Documenter into behaving how I want. Really appreciate the help with the docstrings here

[adknudson]

Happy to help! Documentation is to me one of the most important aspects of a project. It can be the deciding factor for whether a person uses your package or another similar one. I loved the tidyverse when I used R and I'm already enjoying using TidierData.

If it's alright with you, I'll keep working on the documentation and add examples as I go. If there's anything else you think I can help out with, let me know :)

[rdboyes]

It's more than alright! I really appreciate the help with the documentation - as far as other things, I try to keep an active list of things that I think are important to work on in Issues, so if something in there strikes you as something you want to work on, feel free to claim it