Add support for deploying from Buildkite.

[maleadt]

Modeled after GitLab, but I left out the slug and source since they seemed unused anyway.
Example of successful run: https://buildkite.com/julialang/cuda-dot-jl/builds/116

[maleadt]

Looks like this doesn't work on a non-PR:

[Info: �[22m�[39mDeploying to GitHub
┌Info: �[22m�[39m
│Buildkite config:
│  Commit branch: "master"
│  Pull request: "false"
│  Commit tag: ""
│Detected build type: preview
│- ✘ ENV["BUILDKITE_PULL_REQUEST"]="false" is a number
│- ✔ `push_preview` keyword argument to deploydocs is `true`
│- ✔ ENV["DOCUMENTER_KEY"] exists
└Deploying to folder "previews/PR0": ✘�
[36m�[1m[ �[22m�[39mInfo: �[22m�[39mDeploying to GitHub
┌Info: �[22m�[39m
│Buildkite config:
│  Commit branch: "master"
│  Pull request: "false"
│  Commit tag: ""
│Detected build type: preview
│- ✘ ENV["BUILDKITE_PULL_REQUEST"]="false" is a number
│- ✔ `push_preview` keyword argument to deploydocs is `true`
│- ✔ ENV["DOCUMENTER_KEY"] exists
└Deploying to folder "previews/PR0": ✘

I guess I need to filter out the PR if it's set to false.

[mortenpi]

Can't really test it myself, but LGTM. If it works and is good to go, them I'm happy to merge.

[jakebolewski]

LGTM, it would be great to tag a release after this is merged if possible. Currently the Clima project mocks the equiv. Travis CI env variables.

[mortenpi]

it would be great to tag a release after this is merged if possible.

0.26 is still some ways out, but I think we can backport this and a few other PRs and release 0.25.4 over the weekend or so.

[maleadt]

Yes, this works nicely now:

• https://buildkite.com/julialang/cuda-dot-jl/builds/133#327e6fc2-3dc5-4ec0-ba2e-05bb085092ea
• https://github.com/JuliaGPU/CUDA.jl/tree/gh-pages/dev
• https://juliagpu.github.io/CUDA.jl/dev/