Document the official distribution locations and the recommendation to use a built-in copy

[stmontgomery]

This adds a new documentation article within the top-level Documentation/ directory describing the various ways Swift Testing is distributed and offers the specific recommendation that most users prefer using a built-in copy instead of a package dependency.

View the rendered document for more details.

Note: In this PR the new article is only being placed in the Documentation/ directory. I chose to place it there, initially, and not within the project's official DocC documentation catalog, but that could potentially happen in the future.

Motivation:

We have been casually giving users the recommendation to prefer built-in copies of the Testing module for a while in settings like the Swift forums, bug reports, and informal chats. But thus far, we have not collected and formally documented the reasons why using a package copy can encounter issues. This discussion recently came up during a discussion about whether to adopt Swift Testing in a test-helper library in SwiftSyntax.

My hope is that adding this document will help clarify expectations around the level of support we offer for the two distribution styles for Swift Testing.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[heckj]

spotted a couple of typos