Reconfigure ET documentation orders (#3736) (#3738)

Summary:
Pull Request resolved: #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
(cherry picked from commit 01ae21d)

Co-authored-by: Mergen Nachin <mnachin@meta.com>

docs/source/index.rst

@@ -70,26 +70,20 @@ Topics in this section will help you get started with ExecuTorch.
    :hidden:
 
    intro-overview
-   concepts
    intro-how-it-works
+   getting-started-architecture
+   concepts
 
 .. toctree::
    :glob:
    :maxdepth: 1
    :caption: Getting Started
    :hidden:
 
-   getting-started-architecture
    getting-started-setup
+   export-overview
    runtime-build-and-cross-compilation
 
-.. toctree::
-   :glob:
-   :maxdepth: 2
-   :caption: Working with LLMs
-   :hidden:
-
-   llm/getting-started
 
 .. toctree::
    :glob:
@@ -116,11 +110,11 @@ Topics in this section will help you get started with ExecuTorch.
 
 .. toctree::
    :glob:
-   :maxdepth: 1
-   :caption: Exporting to ExecuTorch
+   :maxdepth: 2
+   :caption: Working with LLMs
    :hidden:
 
-   export-overview
+   llm/getting-started
 
 .. toctree::
    :glob: