[CI] Migrate CI pipelines from Jenkins to BuildKite (#8142)

* [CI] Migrate CI pipelines from Jenkins to BuildKite

* Require manual approval

* Less verbose output when pulling Docker

* Remove us-east-2 from metadata.py

* Add documentation

* Add missing underscore

* Add missing punctuation

* More specific instruction

* Better paragraph structure

tests/buildkite/pipeline-win64.yml

@@ -0,0 +1,22 @@
+steps:
+  - block: ":rocket: Run this test job"
+  #### -------- BUILD --------
+  - label: ":windows: Build XGBoost for Windows with CUDA"
+    command: "tests/buildkite/build-win64-gpu.ps1"
+    key: build-win64-gpu
+    agents:
+      queue: windows-gpu
+  - label: ":windows: Build XGBoost R package for Windows with CUDA"
+    command: "tests/buildkite/build-rpkg-win64-gpu.ps1"
+    key: build-rpkg-win64-gpu
+    agents:
+      queue: windows-gpu
+
+  - wait
+
+  #### -------- TEST --------
+  - label: ":windows: Test XGBoost on Windows"
+    command: "tests/buildkite/test-win64-gpu.ps1"
+    key: test-win64-gpu
+    agents:
+      queue: windows-gpu