Add CI action to build docs (flashlight#1131)

Summary:
Test that documentation successfully builds in CI. This will harden against env changes in rtd and will ensure people can't commit broken docs.

Pull Request resolved: flashlight#1131

Test Plan: CI

Reviewed By: EntilZha

Differential Revision: D46667736

Pulled By: jacobkahn

fbshipit-source-id: 03c288727b74b351323631ea13e99f4ba9b62416

.github/workflows/build_docs.yml

@@ -0,0 +1,20 @@
+name: readthedocs/actions
+on:
+  pull_request_target:
+    types:
+      - opened
+    # Execute this action only on PRs that touch
+    # documentation files.
+    # paths:
+    #   - "docs/**"
+
+permissions:
+  pull-requests: write
+
+jobs:
+  pull-request-links:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: readthedocs/actions/preview@v1
+        with:
+          project-slug: "fl"