Replace Buildkite CI with GitHub Actions self-hosted GPU runners

- Remove `.buildkite/` (documentation and aggregate builds on Buildkite)
- Add `.github/workflows/Documentation.yml` with two jobs:
  - Documentation build on `[self-hosted, Linux, X64, gpu]` runners
  - Aggregate documentation build + S3 deploy on self-hosted GPU runners
- AWS and DOCUMENTER_KEY secrets must be configured as GitHub repository secrets

Part of the SciML-wide migration from Buildkite to GitHub Actions.

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

Author: ChrisRackauckas

.buildkite/0_webui.yml

@@ -0,0 +1,66 @@
+name: "Documentation"
+
+on:
+  pull_request:
+    branches:
+      - main
+  push:
+    branches:
+      - main
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: ${{ github.ref_name != github.event.repository.default_branch || github.ref != 'refs/tags/v*' }}
+
+jobs:
+  docs:
+    name: "Build Documentation"
+    runs-on: [self-hosted, Linux, X64, gpu]
+    timeout-minutes: 2000
+    env:
+      DATADEPS_ALWAYS_ACCEPT: true
+      JULIA_DEBUG: "Documenter"
+      JULIA_PKG_SERVER: ""
+    steps:
+      - uses: actions/checkout@v6
+      - uses: julia-actions/setup-julia@v2
+        with:
+          version: "1.11"
+      - name: "Build Documentation"
+        run: |
+          git remote set-branches origin 'gh-pages'
+          git fetch --depth=1 origin gh-pages
+          julia --project -e '
+            println("--- Instantiating project")
+            using Pkg
+            Pkg.instantiate()
+            Pkg.activate("docs")
+            Pkg.instantiate()
+            push!(LOAD_PATH, @__DIR__)
+            println("+++ Building documentation")
+            include("docs/make.jl")'
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          DOCUMENTER_KEY: ${{ secrets.DOCUMENTER_KEY }}
+
+  aggregate:
+    name: "Build and Deploy Aggregate Documentation"
+    runs-on: [self-hosted, Linux, X64, gpu]
+    timeout-minutes: 2000
+    env:
+      JULIA_PKG_SERVER: ""
+    steps:
+      - uses: actions/checkout@v6
+      - uses: julia-actions/setup-julia@v2
+        with:
+          version: "1.11"
+      - name: "Build and Deploy"
+        run: |
+          julia --version
+          julia --project=docs/ -e 'using Pkg; Pkg.develop(PackageSpec(path=pwd())); Pkg.instantiate()'
+          julia --project=docs/ docs/make_aggregate.jl
+          aws s3 sync --delete --acl=public-read docs/build s3://docs.sciml.ai --cache-control max-age=600
+        env:
+          AWS_ACCESS_KEY_ID: ${{ secrets.AWS_ACCESS_KEY_ID }}
+          AWS_SECRET_ACCESS_KEY: ${{ secrets.AWS_SECRET_ACCESS_KEY }}
+          DOCUMENTER_KEY: ${{ secrets.DOCUMENTER_KEY }}