Update and reformat FILE_STRUCTURE.md to include explanations of new files

[Milind220]

FILE_STRUCTURE.md is supposed to explain to new contributors the overall structure of Ozone. This is so that they feel more comfortable moving from file to file to make their contributions.

Currently, it's been a little while since FILE_STRUCTURE was updated. I'm also not too impressed with the current organisation of the file.

Here's the Issue:

• Add description of historical directory and the files within
• Add description of ci/cd directory
• Reformat the file to make it clearer to read

Please ask away if you're not sure what of the purpose of any file! We're more than happy to help you contribute :)