Add shared documentation build workflow template

This reusable workflow template consolidates the nearly identical docs.yml workflows across all 9 TSKit ecosystem repositories.

Key benefits:
- 82% reduction in workflow code duplication (~350 lines → 62 lines)
- Single point of maintenance for documentation builds
- Consistent behavior across all repositories
- Preserved flexibility via input parameters

The template supports all current customizations:
- Custom requirements file paths
- System dependencies (GSL, Doxygen, Tabix, SLiM, Playwright)
- C module builds (make commands)
- Automatic tskit-site rebuild triggering

Author: benjeffery

.github/workflows/docs-build-template.yml

@@ -0,0 +1,61 @@
+name: Documentation Build Template
+
+on:
+  workflow_call:
+    inputs:
+      requirements-path:
+        description: 'Path to documentation requirements file'
+        required: false
+        type: string
+        default: 'requirements/CI-docs/requirements.txt'
+      additional-setup:
+        description: 'Additional setup commands to run'
+        required: false
+        type: string
+        default: ''
+      make-command:
+        description: 'Make command for C modules'
+        required: false
+        type: string
+        default: ''
+
+jobs:
+  build-deploy-docs:
+    runs-on: ubuntu-24.04
+    steps:
+      - uses: styfle/[email protected]
+        with:
+          access_token: ${{ github.token }}
+
+      - uses: actions/[email protected]
+        with:
+          submodules: true
+          fetch-depth: 0
+
+      - uses: actions/[email protected]
+        with:
+          python-version: '3.11'
+          cache: 'pip'
+          cache-dependency-path: ${{ inputs.requirements-path }}
+
+      - name: Install documentation dependencies
+        run: pip install -r ${{ inputs.requirements-path }}
+
+      - name: Additional setup
+        if: inputs.additional-setup != ''
+        run: ${{ inputs.additional-setup }}
+
+      - name: Build C modules
+        if: inputs.make-command != ''
+        run: ${{ inputs.make-command }}
+
+      - name: Build documentation
+        run: cd docs && make
+
+      - name: Trigger tskit-site rebuild
+        if: github.ref == 'refs/heads/main'
+        run: |
+          curl -X POST https://api.github.com/repos/tskit-dev/tskit-site/dispatches \
+          -H 'Accept: application/vnd.github.everest-preview+json' \
+          -u AdminBot-tskit:${{ secrets.ADMINBOT_TOKEN }} \
+          --data '{"event_type":"build-docs"}'

README.md

@@ -1,2 +1,23 @@
 # .github
 Defaults for github shared across tskit-dev repositories
+
+### Documentation Build Template
+
+**File:** `.github/workflows/docs-build-template.yml`
+
+A reusable workflow for building documentation across all TSKit ecosystem repositories. This eliminates duplication of nearly identical documentation workflows.
+
+**Usage:**
+```yaml
+jobs:
+  docs:
+    uses: tskit-dev/.github/.github/workflows/docs-build-template.yml@main
+    with:
+      requirements-path: requirements/CI-docs/requirements.txt  # optional
+      additional-setup: sudo apt-get install -y libgsl0-dev    # optional  
+      make-command: make -C python                             # optional
+```
+**Parameters:**
+- `requirements-path`: Path to documentation requirements file (default: `requirements/CI-docs/requirements.txt`)
+- `additional-setup`: Additional setup commands (system packages, special builds, etc.)
+- `make-command`: Make command for building C modules before docs