Implement docs generation pipeline on Buildkite (#1114)
* Implement docs generation pipeline on Buildkite
After this change we can replace our CloudBuild pipelines with Buildkite pipelines. They'll run as GitHub postsubmit and create and upload new versions of the Bazel documentation, the blog and the website.
Moreover, this code will be used by a "docs staging" presubmit pipeline, which still needs some work, though.
Next steps:
1. Create Buildkite pipelines
2. Disable CloudBuild pipelines
3. Enable Buildkite pipelines
4. Update (internal) documentation
5. Fix and enable staging pipeline
Progress towards #1110 (Milestone 1)
5 files changed
