docs: write guide on adding new test suites to circleci

[ericlaz]

Motivation

This PR aims to explain how to add new test suites in the contributing docs. This can be highly useful for new contributors as a reference to follow when writing new integrations for dd-trace-py.

Checklist

• Change(s) are motivated and described in the PR description.
• Testing strategy is described if automated tests are not included in the PR.
• Risk is outlined (performance impact, potential for breakage, maintainability, etc).
• Change is maintainable (easy to change, telemetry, documentation).
• Library release note guidelines are followed. If no release note is required, add label changelog/no-changelog.
• Documentation is included (in-code, generated user docs, public corp docs).
• Backport labels are set (if applicable)

Reviewer Checklist

• Title is accurate.
• No unnecessary changes are introduced.
• Description motivates each change.
• Avoids breaking API changes unless absolutely necessary.
• Testing strategy adequately addresses listed risk(s).
• Change is maintainable (easy to change, telemetry, documentation).
• Release note makes sense to a user of the library.
• Reviewer has explicitly acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment.
• Backport labels are set in a manner that is consistent with the release branch maintenance policy
• If this PR touches code that signs or publishes builds or packages, or handles credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

[emmettbutler]

This is awesome, thanks for taking the time to think about how to make documentation useful. I suggested a few changes that are pretty minor.

[pr-commenter]

Benchmarks

Benchmark execution time: 2023-08-28 21:04:32

Comparing candidate commit ef399ba in PR branch eric.navarro/update-contributing-docs with baseline commit 056f0a3 in branch 1.x.

Found 1 performance improvements and 0 performance regressions! Performance is the same for 103 metrics, 0 unstable metrics.

scenario:flasksimple-appsec-get

• 🟩 max_rss_usage [-831.762KB; -693.179KB] or [-2.436%; -2.030%]