Prepare a simple TOC for 4/2 launch. (#2728)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2728

I think the current TOC contains more information that would be later populated after 4/2.

I simplified the current TOC to be one page and reflecting current blockers for 4/2 launch. We can always venture out to more pages, but let's start with a simple single page.

Reviewed By: dbort

Differential Revision: D55451176

fbshipit-source-id: dc49460fdfbaa0f9b242671b3c9a69e412b1b899