TEST-52 Add impact-key to the adaptive testing docs

[liamclarkedev]

Description

To prevent the files and tests conflicting with other in the same project (e.g. service-1/index.test.ts, service-2/index.test.ts), the impact-key option can be passed to group relevant impact data together when running analysis.

Reasons

Why did you make these changes?

To document a new addition to the adaptive testing feature.

Content Checklist

Please follow our style when contributing to CircleCI docs. Our style guide is here: https://circleci.com/docs/style/style-guide-overview.

Please take a moment to check through the following items when submitting your PR (this is just a guide so will not be relevant for all PRs):

• Break up walls of text by adding paragraph breaks.
• Consider if the content could benefit from more structure, such as lists or tables, to make it easier to consume.
• Keep the title between 20 and 70 characters.
• Consider whether the content would benefit from more subsections (h2-h6 headings) to make it easier to consume.
• Check all headings h1-h6 are in sentence case (only first letter is capitalized).
• Include relevant backlinks to other CircleCI docs/pages.

[liamclarkedev]

Updated to include config option and include a config example.