Make docs build less flaky (#420)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/420

The docs build sometimes fails with an error like

```
mv docs/_build/html /artifacts
mv: inter-device move failed: 'docs/_build/html' to '/artifacts/html'; unable to remove target: Directory not empty
```
(from https://github.com/pytorch/executorch/actions/runs/6251991315/job/16974762713#step:11:209)

Update the script to force-remove any existing `/artifacts/html` directory before copying, which should avoid this error.
ghstack-source-id: 201361283
exported-using-ghexport

Reviewed By: huydhn, svekars

Differential Revision: D49466514

fbshipit-source-id: 09a11ccfda57109a640a4b15eac48c0338e406c5