Split up the main README

[adhilto]

💡 Summary

Split up the main README into separate files.

Motivation and context

The README is growing large and unwieldy. Splitting it into smaller files will improve readability and maintainability.

Implementation notes

See ScubaGear's README for reference: https://github.com/cisagov/ScubaGear/tree/main

Acceptance criteria

• [ ] The README has been split into different files, using a structure similar to ScubaGear's documentation.

[LaurenBassett]

Linking #283 since it is a readme related issue