Reconfigure ET documentation orders (#3736)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/3736

- Demote "ExecuTorch Concepts" page to the back of "Intro" card
- Move "High-level architecture" from "Getting Started" to "Introduction"
- Remove a dedicated Export section and move that one page to "Getting Started"
- Demote "Working with LLMs" to after "Tutorials" section.

https://pytorch.org/executorch/stable/index.html

Reviewed By: dbort

Differential Revision: D57792707

fbshipit-source-id: 238b42f5caaeb85b8e6022cc6fb9045d2d4ea9cc