Add publishing workflow (just-the-docs#172)

Publish to RubyGems and GPR on release

Author: pmarsceill

.github/workflows/publish.yml

@@ -0,0 +1,39 @@
+name: Publish Gem
+
+on: [release]
+
+jobs:
+  build:
+    name: Build + Publish
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@master
+    - name: Set up Ruby 2.6
+      uses: actions/setup-ruby@v1
+      with:
+        version: 2.6.x
+
+    - name: Publish to GPR
+      run: |
+        mkdir -p $HOME/.gem
+        touch $HOME/.gem/credentials
+        chmod 0600 $HOME/.gem/credentials
+        printf -- "---\n:github: Bearer ${GEM_HOST_API_KEY}\n" > $HOME/.gem/credentials
+        gem build just-the-docs.gemspec
+        gem push --KEY github --host https://rubygems.pkg.github.com/${OWNER} *.gem
+      env:
+        GEM_HOST_API_KEY: ${{secrets.GPR_AUTH_TOKEN}}
+        OWNER: pmarsceill
+
+    - name: Publish to RubyGems
+      run: |
+        mkdir -p $HOME/.gem
+        touch $HOME/.gem/credentials
+        chmod 0600 $HOME/.gem/credentials
+        printf -- "---\n:rubygems_api_key: ${GEM_HOST_API_KEY}\n" > $HOME/.gem/credentials
+        gem build just-the-docs.gemspec
+        gem push *.gem
+      env:
+        GEM_HOST_API_KEY: ${{secrets.RUBYGEMS_AUTH_TOKEN}}
+