updated getting started (#9238)

Update of the Getting Started guide for Swift Package Manager

### Motivation:

The existing getting started guide focused entirely on dependency
resolution, with content that pre-dates swift testing, dependency CLI
additions, and common use of templates to start packages.

### Modifications:

Revised the getting started guide article, using recent updates to
example packages that were previously created, and recently updated to
reflect swift-testing and use Swift 6.0 by default.

### Result:

- focus on starting with `swift package init`
- shows command line additions of dependencies alongside changes to
Package.swift manifest file
- include source AND tests (using swift-testing)
- walk through `swift build`, `swift test`, and `swift package resolve`
- illustrate using transitive dependencies using the existing dealer
example package
- show using `swift run` for executable targets

Author: heckj