[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/conftest.ps1

@@ -0,0 +1,11 @@
+if ( $Env:BUILDKITE_PULL_REQUEST -and ($Env:BUILDKITE_PULL_REQUEST -ne "false") ) {
+  $is_pull_request = 1
+} else {
+  $is_pull_request = 0
+}
+
+if ( ($Env:BUILDKITE_BRANCH -eq "master") -or ($Env:BUILDKITE_BRANCH -match "release_.+") ) {
+  $is_release_branch = 1
+} else {
+  $is_release_branch = 0
+}