[ci] Convert GitHub Actions files to new format

GitHub.com is changing the supported configuration file format for its
"Actions" feature [1]. Update to the new format.

This change set was produced using the tooling provided by GitHub.com
[2].

[1] > The documentation at https://developer.github.com/actions and
    > support for the HCL syntax in GitHub Actions will be deprecated on
    > September 30, 2019. Documentation for the new limited public beta
    > using the YAML syntax is available on https://help.github.com. See
    > "Automating your workflow with GitHub Actions" for documentation
    > using the YAML syntax.

    https://developer.github.com/actions/
[2] https://github.com/actions/migrate

.github/workflows/pull_request.yml

@@ -0,0 +1,14 @@
+on: pull_request
+name: Synchronize the Pull Request Preview
+jobs:
+  update-pr-preview:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@master
+    - name: update-pr-preview
+      uses: ./tools/docker/github
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      with:
+        entrypoint: python
+        args: tools/ci/update_pr_preview.py https://api.github.com

.github/workflows/push-build-publish-documentation-website.yml

@@ -0,0 +1,14 @@
+on: push
+name: Build & Publish Documentation Website
+jobs:
+  website-build-and-publish:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@master
+    - name: website-build-and-publish
+      uses: ./tools/docker/documentation
+      env:
+        DEPLOY_TOKEN: ${{ secrets.DEPLOY_TOKEN }}
+      with:
+        entrypoint: /bin/bash
+        args: tools/ci/website_build.sh

.github/workflows/push-build-release-manifest.yml

@@ -0,0 +1,14 @@
+on: push
+name: Build & Release Manifest
+jobs:
+  manifest-build-and-tag:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@master
+    - name: manifest-build-and-tag
+      uses: ./tools/docker/github
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      with:
+        entrypoint: python
+        args: tools/ci/manifest_build.py