ci: add standard centralized workflows (auto-format, dependabot-automerge, docs-preview-cleanup) (#55)

Adds the standard SciML centralized caller workflows that were missing:
- RunicSuggestions.yml (Runic auto-format suggestions on PRs)
- DependabotAutoMerge.yml (auto-merge Dependabot PRs)
- DocPreviewCleanup.yml (clean up doc previews on PR close)

Co-authored-by: ChrisRackauckas-Claude <noreply@anthropic.com>
Co-authored-by: Chris Rackauckas <accounts@chrisrackauckas.com>

Author: 3 people

.github/workflows/DependabotAutoMerge.yml

@@ -0,0 +1,9 @@
+name: "Dependabot Auto-merge"
+on: pull_request
+permissions:
+  contents: write
+  pull-requests: write
+jobs:
+  automerge:
+    uses: "SciML/.github/.github/workflows/dependabot-automerge.yml@v1"
+    secrets: "inherit"

.github/workflows/DocPreviewCleanup.yml

@@ -0,0 +1,10 @@
+name: "Doc Preview Cleanup"
+on:
+  pull_request:
+    types: [closed]
+permissions:
+  contents: write
+jobs:
+  cleanup:
+    uses: "SciML/.github/.github/workflows/docs-preview-cleanup.yml@v1"
+    secrets: "inherit"

.github/workflows/RunicSuggestions.yml

@@ -0,0 +1,9 @@
+name: "Runic Suggestions"
+on: pull_request
+permissions:
+  contents: read
+  pull-requests: write
+jobs:
+  runic-suggestions:
+    uses: "SciML/.github/.github/workflows/runic-suggestions-on-pr.yml@v1"
+    secrets: "inherit"