Add an "Openwater Repository Guide" describing how the repositories fit together

Problem

As a new contributor, it takes some time to understand how the different Openwater repositories relate to one another.

The documentation currently covers OpenLIFU and OpenMotion independently, but there doesn't appear to be a single page explaining the overall repository ecosystem and where different types of work belong.

For example:

• Which repository contains the SDK?
• Which repository contains the operator or clinical application?
• Where should documentation changes go?
• Which repositories are libraries versus end-user applications?

Proposal

Add a short "Repository Guide" page to the documentation that introduces the major repositories and how they relate.

For example:

OpenMotion

openmotion-sdk
        ↓
openmotion-bloodflow-app

OpenLIFU

openlifu-sdk
        ↓
openlifu-operator-interface

Documentation

openwater-docs

Each repository could have a brief description such as:

Optionally, the page could also include links to each repository's README and contribution guidelines.

Motivation

This would help new contributors quickly understand:

• where new features belong;
• where to report bugs;
• where to submit documentation changes;
• how the SDKs and applications relate to one another.

I think it would make onboarding considerably easier without changing any existing documentation structure.