Updated Documentation

[ansengarvin]

• Split the database section of backend.md into its own document, database.md
  • backend.md links to database.md
  • Content of database documentation is the same, but formatted slightly differently
• Created new documentation in api.md
  • This aims to create a simple step-by-step tutorial on creating a new API endpoint, with the goal of showing a new developer how to make one.
  • It uses some content from (and links to) overview.md, so this may end up being redundant.
• Updated run.md documentation with some commands that weren't added.

Closes #252