Create stale.yml (#721)

Summary:
Github action to close issues/PRs that are not labelled with "enhancement" or "how-to" if they have not had any activity in 30 days.

Pull Request resolved: https://github.com/facebookresearch/pytorch3d/pull/721

Reviewed By: bottler

Differential Revision: D29309817

Pulled By: nikhilaravi

fbshipit-source-id: 039f6d4f29e9d04c01f59d145317a34ad75026a5

.github/workflows/stale.yml

+name: Mark stale issues and pull requests
+
+on:
+  schedule:
+  - cron: '31 5 * * *'
+
+jobs:
+  stale:
+
+    runs-on: ubuntu-latest
+    permissions:
+      issues: write
+      pull-requests: write
+
+    steps:
+    - uses: actions/stale@v3
+      with:
+        repo-token: ${{ secrets.GITHUB_TOKEN }}
+        exempt-issue-labels: 'enhancement,how-to'
+        stale-issue-message: 'This issue is stale because it has been open 30 days with no activity. Remove stale label or comment or this will be closed in 5 days.'
+        stale-pr-message: 'This PR is stale because it has been open 45 days with no activity. Remove stale label or comment or this will be closed in 10 days.'
+        close-issue-message: 'This issue was closed because it has been stalled for 5 days with no activity.'
+        close-pr-message: 'This PR was closed because it has been stalled for 10 days with no activity.'
+        days-before-issue-stale: 30
+        days-before-pr-stale: 45
+        days-before-issue-close: 5
+        days-before-pr-close: 10