docs: clarify OCI artifact ordering in DHI customize Docker Hub flow

[lacatoire]

Fixes #24819.

In content/manuals/dhi/how-to/customize.md the Docker Hub tab jumped from "select the repository / select the tag" straight to "images later in the list will overwrite files from earlier images" without first establishing that multiple OCI artifacts can be added or how their ordering is determined in the UI. Readers understood the consequence but not the mechanism.

Adds one short paragraph before the overwrite rule to anchor the reader:

• multiple OCI artifacts can be added to a single customization,
• they're applied in the order they're added in the OCI artifacts drop-down.

The overwrite rule is rephrased to:

• refer to "images added later / images added earlier" for consistency with the new paragraph,
• use "If multiple images contain directories or files with the same path" (the rule applies to any N≥2, not just two),
• use "directories or files" instead of the existing "directories and files", which could be read as requiring both types in the same artifact for the rule to apply.

No change to the subsequent paragraphs about include/exclude paths.

[netlify]

✅ Deploy Preview for docsdocker ready!

Name	Link
🔨 Latest commit	2f2a01c
🔍 Latest deploy log	https://app.netlify.com/projects/docsdocker/deploys/69e6521196abcd0008c1c62c
😎 Deploy Preview	https://deploy-preview-24836--docsdocker.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[craig-osterhout]

LGTM. Thanks.