Migrate from Travis CI to GitHub Actions (#1695)

Summary:
We've previously had the docs deployment in Travis. Also some builds
but they were redundant, so that doesn't really matter.

Pull Request resolved: https://github.com/facebook/flipper/pull/1695

Test Plan:
Tested it on my own branch.
6ac5b3fa70

![Screenshot 2020-11-20 at 11 09 34](https://user-images.githubusercontent.com/9906/99795354-8cbd5480-2b23-11eb-9fe9-10504b19e6a4.png)

Reviewed By: nikoant

Differential Revision: D25119924

Pulled By: passy

fbshipit-source-id: b31ccd05daacc7c0a9a452e88638ba9a3bcc4c74

.github/workflows/docs.yml

@@ -0,0 +1,28 @@
+name: Docs
+on:
+  push:
+    branches:
+      - master
+
+jobs:
+  build-and-deploy:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2.3.1
+        with:
+          persist-credentials: false
+
+      - name: Install and Build
+        run: |
+          yarn
+          yarn  build
+        working-directory: website/
+
+      - name: Deploy
+        uses: JamesIves/github-pages-deploy-action@3.7.1
+        with:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          BRANCH: gh-pages
+          FOLDER: website/build
+          CLEAN: true