docs: add Buildkite example to CI Vendors documentation (#10721)
I've created some documentation that you can use, if interested in
merging, regarding how you can use Buildkite as CI vendor with
Turborepo. Also added Buildkite to the dictionary to make the PR linter
safe.
Testing Instructions:
1. Checkout PR branch.
2. Run local instance of Turborepo documentation.
3. Navigate to this edited page:
https://turborepo.com/docs/guides/ci-vendors
4. Navigate to this new page:
https://turborepo.com/docs/guides/ci-vendors/buildkite
<img width="976" height="320" alt="image"
src="https://github.com/user-attachments/assets/71f215e6-6531-4023-b9a7-3ee055b642d6"
/>
<img width="992" height="375" alt="image"
src="https://github.com/user-attachments/assets/a6e71835-0111-4637-a87e-cc02322f4f18"
/>
---------
Co-authored-by: Christian Nunciato <chris@nunciato.org>
