Make docs build use the install pip package (#2678)

Summary:
Before now, the docs execution environment was adding the source tree to the import path instead of using the installed pip files.

Also, it added the `executorch` directory, making it possible to refer to packages like `sdk.bundled_program` instead of
`executorch.sdk.bundled_program`.

Stop adding the source directories to the path, and fix up imports.

Pull Request resolved: https://github.com/pytorch/executorch/pull/2678

Test Plan:
```
cd docs/source
make html
```

Reviewed By: Jack-Khuu

Differential Revision: D55351005

Pulled By: dbort

fbshipit-source-id: 2331bac1fbb751f60858d50e833a7500fc2081d4