docs(external-router): clarify README and getting started guides

[damienwebdev]

PR Checklist

• [x] Commit message follows our contributing guidelines
• [x] Tests added/updated (for bug fixes/features)
• [ ] Documentation added/updated (for bug fixes/features)

PR Type

• [ ] Bug fix
• [ ] Feature
• [ ] Style update
• [ ] Refactor
• [ ] Test
• [ ] Build
• [ ] CI
• [x] Docs
• [ ] Performance
• [ ] Other (please describe)

Current behavior

I tried to set up the external router driver by following the docs and I was a little confused, so I had to look at the code to remember how it worked.

Touches: https://github.com/graycoreio/daffodil/issues/2683

New behavior

I rewrote the guide to be a little more clear on how to use the package.

Breaking change?

• [ ] Yes
• [x] No

Additional context