Update docs-preview link

[nkammah]

Following the migration from Jenkins to Buildkite, docs previews are now available at bk.

More context in elastic/docs#2898

Note - a shared GH action can also be found here: https://github.com/elastic/docs/tree/master/.github/actions/docs-preview

• Have you signed the contributor license agreement?
• Have you followed the contributor guidelines?
• For proposing substantial changes or additions to the schema, have you reviewed the RFC process?
• If submitting code/script changes, have you verified all tests pass locally using make test?
• If submitting schema/fields updates, have you generated new artifacts by running make and committed those changes?
• Is your pull request against main? Unless there is a good reason otherwise, we prefer pull requests against main and will backport as needed.
• Have you added an entry to the CHANGELOG.next.md?