Replace Buildkite CI with GitHub Actions self-hosted runners

Migrate CUDA tests and documentation builds from Buildkite to GitHub
Actions using self-hosted runners with the `gpu` label (Tesla T4 on arctic1).

- Add .github/workflows/GPU.yml with CUDA tests (240min) and docs build (360min)
- Delete .buildkite/pipeline.yml

Part of the SciML-wide migration from Buildkite to GitHub Actions
self-hosted runners. See ChrisRackauckas/InternalJunk#16
for runner infrastructure details.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ChrisRackauckas

.buildkite/pipeline.yml

@@ -0,0 +1,62 @@
+name: "GPU Tests"
+
+on:
+  pull_request:
+    branches:
+      - master
+      - 'release-'
+    paths-ignore:
+      - 'docs/**'
+  push:
+    branches:
+      - master
+    paths-ignore:
+      - 'docs/**'
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: ${{ github.ref_name != github.event.repository.default_branch || github.ref != 'refs/tags/v*' }}
+
+jobs:
+  cuda-tests:
+    name: "CUDA Tests"
+    runs-on: [self-hosted, Linux, X64, gpu]
+    timeout-minutes: 240
+    steps:
+      - uses: actions/checkout@v6
+      - uses: julia-actions/setup-julia@v2
+        with:
+          version: "1"
+      - uses: julia-actions/julia-buildpkg@v1
+      - uses: julia-actions/julia-runtest@v1
+        env:
+          GROUP: "CUDA"
+      - uses: julia-actions/julia-processcoverage@v1
+      - uses: codecov/codecov-action@v5
+        with:
+          files: lcov.info
+          token: ${{ secrets.CODECOV_TOKEN }}
+          fail_ci_if_error: false
+
+  gpu-docs:
+    name: "Documentation"
+    runs-on: [self-hosted, Linux, X64, gpu]
+    timeout-minutes: 360
+    if: github.event_name == 'push' || !github.event.pull_request.draft
+    steps:
+      - uses: actions/checkout@v6
+      - uses: julia-actions/setup-julia@v2
+        with:
+          version: "1"
+      - run: |
+          julia --project=docs -e '
+            println("--- Instantiating project")
+            using Pkg
+            Pkg.develop(PackageSpec(path=pwd()))
+            Pkg.instantiate()
+            println("+++ Building documentation")
+            include("docs/make.jl")'
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          DOCUMENTER_KEY: ${{ secrets.DOCUMENTER_KEY }}
+          DATADEPS_ALWAYS_ACCEPT: true