docs: Add documentation on repo folder structure

[kimadactyl]

Checklist:

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation

Fixes #2631

Description

Adds a section describing the current contents of the code base

Motivation and Context

We are currently working to make the codebase more maintainable, so it makes sense to start with an audit of what's there now.

Future updates to the folder structure will need to remember to update this file.

Type of change

• Documentation update

_{Huly®: PC-2629}

[lenikadali]

Nice work 🙌🏿

Only suggestion I have is to check the Rails guides across versions before deleting anything so that it doesn't bite us later 😅 But that shouldn't stop this from being merged 😇

[kimadactyl]

Nice work 🙌🏿

Only suggestion I have is to check the Rails guides across versions before deleting anything so that it doesn't bite us later 😅 But that shouldn't stop this from being merged 😇

I am hoping the tests stop anything bad happening!