Add GitHub Actions workflow to build book and push to github pages

- Run book build on all commits
- Deploy site on pushes to master

type: development

.github/workflows/site.yaml

@@ -0,0 +1,28 @@
+name: Site
+
+on:
+  push:
+    branches:
+    - master
+  pull_request:
+    branches:
+    - master
+
+jobs:
+  all:
+    name: All
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Build
+        uses: peaceiris/actions-mdbook@v1
+        with:
+          mdbook-version: latest
+      - run: mdbook build book --dest-dir ../docs/book
+      - name: Deploy
+        uses: peaceiris/actions-gh-pages@v3
+        if:   github.ref == 'refs/heads/master'
+        with:
+          github_token:   ${{secrets.GITHUB_TOKEN}}
+          publish_branch: gh-pages
+          publish_dir:    ./docs