GitHub Workflows security hardening (#870)

Summary:
This PR adds explicit [permissions section](https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#permissions) to workflows. This is a security best practice because by default workflows run with [extended set of permissions](https://docs.github.com/en/actions/security-guides/automatic-token-authentication#permissions-for-the-github_token) (except from `on: pull_request` [from external forks](https://securitylab.github.com/research/github-actions-preventing-pwn-requests/)). By specifying any permission explicitly all others are set to none. By using the principle of least privilege the damage a compromised workflow can do (because of an [injection](https://securitylab.github.com/research/github-actions-untrusted-input/) or compromised third party tool or action) is restricted.
It is recommended to have [most strict permissions on the top level](https://github.com/ossf/scorecard/blob/main/docs/checks.md#token-permissions) and grant write permissions on [job level](https://docs.github.com/en/actions/using-jobs/assigning-permissions-to-jobs) case by case.

Pull Request resolved: #870

Reviewed By: jacdebug

Differential Revision: D42406481

Pulled By: robhogan

fbshipit-source-id: c1118141c8b95d85c850ebe91b6b72c3b5170869

.github/workflows/deploy-website.yml

@@ -9,8 +9,13 @@ on:
       - "website/**"
       - ".github/workflows/deploy-website.yml"
 
+permissions:
+  contents: read
+
 jobs:
   deploy:
+    permissions:
+      contents: write # to deploy
     runs-on: ubuntu-latest
 
     if: github.repository == 'facebook/metro'