Dead link on README.md

[stuartpa]

https://github.com/Azure/data-api-builder/blob/main/docs/getting-started/getting-started.md

[beanscg]

Verified this against current main:

• Source: README.md:16 still points "Getting Started" at https://github.com/Azure/data-api-builder/blob/main/docs/getting-started/getting-started.md.
• Current behavior: that GitHub path returns HTTP 404.
• Current Microsoft Learn target: https://learn.microsoft.com/azure/data-api-builder/get-started/get-started-with-data-api-builder resolves to the current basic SQL quickstart and returns HTTP 200.
• Duplicate check: the open PRs I saw are unrelated sample/dependency changes and do not appear to cover this README link.

Smallest fix looks like replacing the old engine-repo Markdown path with the current Microsoft Learn quickstart URL.