docs: directly mention CI for new integrations [LANGPLAT-352]

[emmettbutler]

What does this PR do?
This change makes it explicit that the given instructions on making a new integration are enough to get its tests to run in CI.

Motivation:
To make it easier for new contributors to become effective

Change log entry

Additional Notes:

How to test the change?

[github-actions]

👋 Hey @DataDog/ruby-guild, please fill "Change log entry" section in the pull request description.

If changes need to be present in CHANGELOG.md you can state it this way

**Change log entry**

Yes. A brief summary to be placed into the CHANGELOG.md

(possible answers Yes/Yep/Yeah)

Or you can opt out like that

**Change log entry**

None.

(possible answers No/Nope/None)

^{Visited at: 2025-06-26 20:10:15 UTC}

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.54%. Comparing base (39ff198) to head (52f3d79).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4763      +/-   ##
==========================================
- Coverage   97.55%   97.54%   -0.01%     
==========================================
  Files        1486     1486              
  Lines       88715    88715              
  Branches     4599     4599              
==========================================
- Hits        86548    86541       -7     
- Misses       2167     2174       +7     

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[datadog-datadog-prod-us1]

Datadog Report

Branch report: emmett.butler/ci-note
Commit report: 52f3d79
Test service: dd-trace-rb

✅ 0 Failed, 21523 Passed, 1295 Skipped, 4m 52.1s Total Time

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-06-26 20:41:28

Comparing candidate commit 52f3d79 in PR branch emmett.butler/ci-note with baseline commit 39ff198 in branch master.

Found 3 performance improvements and 0 performance regressions! Performance is the same for 44 metrics, 2 unstable metrics.

scenario:error - error tracking with http - all

• 🟩 throughput [+12.689op/s; +17.660op/s] or [+10.044%; +13.979%]

scenario:error - error tracking with http - user code only

• 🟩 throughput [+7.205op/s; +10.634op/s] or [+5.498%; +8.114%]

scenario:error - without error tracking with http

• 🟩 throughput [+21.294op/s; +28.222op/s] or [+18.016%; +23.877%]