[Docs] Clarify Co-op Translator attribution in README

[Minseok Song (skytin1004)]

Purpose

Update the README heading to "Translations maintained with Co-op Translator" so it identifies the translation tool without tying it to GitHub Actions or promising translations are always current.

Does this introduce a breaking change?

When developers merge from main and run the server, azd up, or azd deploy, will this produce an error?
If you're not sure, try it out on an old environment.

[ ] Yes
[x] No

Does this require changes to learn.microsoft.com docs?

This repository is referenced by (https://azure.microsoft.com/products/phi-3)
which includes deployment, settings and usage instructions.

[ ] Yes
[x] No

Type of change

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[x] Documentation content changes
[ ] Other... Please describe:

Validation

Validation: git diff --check passed; the generated language-table block is unchanged; linked destinations return HTTP 200. Documentation-only change; translation workflows and generated files are unchanged. No application tests were run.