Update docs site URL from preview.bazel.build to bazel.build

The docs contribution workflow guide in https://github.com/bazelbuild/bazel/pull/29499 will link to [preview.bazel.build](https://preview.bazel.build) on the live docs site when it's merged. Once the domain cutover happens, this should be updated to [bazel.build](https://bazel.build) per https://github.com/bazelbuild/bazel/pull/29499#discussion_r3242901495

## What will need to change

- `docs/contribute/docs-contribution-workflow.mdx` — the table in the "Bazel docs structure" section references `preview.bazel.build`
- Any other places in the docs or README that link to `preview.bazel.build`

## Notes

This is a placeholder issue to track the cutover. No action needed until the domain switch happens.