Create github-pages.yml (#109)

Builds site and commits static files to gh-pages branch on every commit to master branch.

Github pages only allows a small set of plugins. We recently added jekyll-tabs in #95, and will probably add more in the future. If this action works well, will update the site settings to serve what is in gh-pages branch.

.github/workflows/github-pages.yml

@@ -0,0 +1,26 @@
+# see https://jekyllrb.com/docs/continuous-integration/github-actions/
+
+name: Build and deploy Jekyll site to GitHub Pages
+
+on:
+  push:
+    branches:
+      - master
+
+jobs:
+  github-pages:
+    runs-on: ubuntu-16.04
+    steps:
+      - uses: actions/checkout@v2
+
+      # Use GitHub Actions' cache to shorten build times and decrease load on servers
+      - uses: actions/cache@v1
+        with:
+          path: vendor/bundle
+          key: ${{ runner.os }}-gems-${{ hashFiles('**/Gemfile') }}
+          restore-keys: |
+            ${{ runner.os }}-gems-
+      
+      - uses: helaili/jekyll-action@2.0.1
+        env:
+          JEKYLL_PAT: ${{ secrets.JEKYLL_PAT }}