Add package guidelines to contributing guidelines

[tobeycodes]

We should add guidelines for how our packages are built to the contributing guidelines. When building requests yesterday I tried to follow onboarding but the conventions are not immediately obvious so we should document them for others to follow.

For example, one guideline I think applies:

• Files within packages should be prefixed with their package name e.g. Onboarding package packages/onboarding/src/onboarding-{type}-{name}.ts

Some questions I'd like clarity on:

• What goes in the data access folder? I see utility functions, hooks, providers, router loaders, redirects, typescript interface files, etc
• What goes in the UI folder?
• When do you choose between the src folder or the ui folder for files that have -ui- in them?
• What is a feature? Why do these not go in a folder features?
• When would you use a feature vs a ui vs neither (e.g. routes)

I do not have much of an opinion on what these conventions should be but do not currently think they are obvious enough to follow