Clean up README

[sarahchen6]

Improve README clarity (and test #139)

[github-actions]

Mirrored test images for 143_merge-* were added in https://github.com/DataDog/images/pull/9085. When you've finished validating the image, please remove the mirrored test images.

[sarahchen6]

After running Create test image mirror PR for 143 (workflow run), this PR to add the mirrored test images was automatically opened: https://github.com/DataDog/images/pull/9085 🙌 It was automatically approved and would've been automatically merged like this PR if I had been more patient by a few more seconds.

[sarahchen6]

After 6fca017 and running the workflow again (workflow run), an update PR was automatically created: https://github.com/DataDog/images/pull/9086 🙌 This was automatically approved and merged.

[sarahchen6]

Tested the 143_merge-* mirrored images in dd-trace-java CI: DataDog/dd-trace-java#10827

[sarahchen6]

After this PR is merged, we can check the Update mirror digests for ci-* images workflow and remove the test images from DataDog/images.

[AlexeyKuznetsov-DD]

LGTM, but maybe make sense to replace 123 with NNN because it is a bit confusing why we referencing to very particular PR number.

[sarahchen6]

Following up, the Update mirror digests for ci-* images workflow succeeded (workflow run) and created this PR: https://github.com/DataDog/images/pull/9088 to update ci-* image digests, which was automatically merged 🙌