Fix the docs build by using unique titles (#561)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/561

Fix an OSS docs job breakage introduced by D49550056. Based on the failure message at https://github.com/pytorch/executorch/actions/runs/6358969960/job/17271725544#step:11:514, it looks like titles need to be unique within a document. I'm guessing it's because they're used to create `#anchor` IDs for section URLs.

ghstack-source-id: 202585556
exported-using-ghexport

Reviewed By: svekars

Differential Revision: D49831895

fbshipit-source-id: b2460e1853f7796b7d09e59f43e42ed5d819c2d8