enhance: Edit the Get Involved/Contribute Documentation doc

[joelhans]

https://athensresearch.gitbook.io/handbook/community/get-involved/learning-at-athens-academy-education/contributing-documentation

https://github.com/athensresearch/handbook/blob/main/community/get-involved/learning-at-athens-academy-education/contributing-documentation.md

Self-assigning this TODO, as I wrote the document in the first place.

Enhancements include:

• Update the list of folders and what they do
• Update the GitBook section with new information
• Add some quickstart items
• Update link from Documentation project -> Handbook project

[joelhans]

Fixed by 1b8ed26