Fix failing doc build on CI due to sphinx version (#1614)

Summary:
By pinning all doc build dependencies like https://github.com/pytorch/pytorch/blob/main/.ci/docker/requirements-docs.txt

Pull Request resolved: https://github.com/pytorch/executorch/pull/1614

Reviewed By: digantdesai

Differential Revision: D52825596

Pulled By: huydhn

fbshipit-source-id: 92ff5f97e1a7d4096103c441a4e2c23f4433400c