Frontend: Populate readme with helpful documentation

[TwoLettuce]

Overview

As you can see, the README in the frontend folder is not particularly helpful. Since documentation is important in facilitating contributions by new TAs, it seems a good idea to have some documentation about the frontend. There's also no need to be excessive with documentation. Some things can only be learned (or are best learned) by looking at the code itself. The readme, along with other relevant documentation, should serve as a catalyst for understanding the web application driving the frontend of the autograder.

Requested Changes

Some important items to document, from my limited understanding:

• links to Vue and TypeScript documentation
• any notable frontend dependencies, with links to their documentation, and the role they play in the application (if not self-evident)
• the linter's purpose, function, and specific things it will change/enforce
• any relevant historical changes
• a summary of the purpose of key files
• a style guide. See Docs: create a style guide #461

Feel free to comment anything else that you would like to know as a new TA. Suggestions about information you know that should be documented would also be helpful if you are experienced with the autograder.