Split out actions, only run them as required, fix GITHUB_SHA under PR context

.github/workflows/publish-docs.yml

@@ -0,0 +1,30 @@
+name: Publish Docs
+on:
+  push:
+    branches:
+      - master
+jobs:
+  docs:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Cache SBT
+        uses: actions/cache@v2
+        with:
+          path: |
+            ~/.ivy2/cache
+            ~/.sbt
+          key: ${{ runner.os }}-sbt-${{ hashFiles('**/build.sbt') }}
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Setup Scala
+        uses: olafurpg/setup-scala@v12
+      - name: Build docs
+        run: sbt docs/unidoc
+      - name: Deploy to GitHub Pages
+        uses: JamesIves/github-pages-deploy-action@4.1.4
+        with:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          BRANCH: gh-pages
+          FOLDER: docs/target/scala-2.13/unidoc