Install Executorch when building docs (#467)

Summary:
This should fix the missing executorch module in https://github.com/pytorch/executorch/actions/runs/6267937461/job/17021962867?pr=437#step:11:254.  Note that all the docs build dependencies are pre-installed in the Docker image with the exception of Executorch, which needs to be installed when the CI runs to get the latest code from the change.

Pull Request resolved: https://github.com/pytorch/executorch/pull/467

Reviewed By: mergennachin

Differential Revision: D49566304

Pulled By: huydhn

fbshipit-source-id: 18701cd8a7bb9624df79a632539411d55f67a978