Moving CMake build documentation to getting started section (#883)

Summary:
Pull Request resolved: #883

It feels like this makes more sense to have in the getting started section as people will want to get setup with CMake early on.

Reviewed By: larryliu0820, cccclai

Differential Revision: D50250770

fbshipit-source-id: ef4691a0396c4aa0ce0ff503080af00d23b7bcbd

Author: tarun292

docs/source/runtime-build-and-cross-compilation.md renamed to docs/source/getting-started-build-and-cross-compilation.md

@@ -78,6 +78,7 @@ Topics in this section will help you get started with ExecuTorch.
 
    getting-started-architecture
    getting-started-setup
+   getting-started-build-and-cross-compilation
 
 .. toctree::
    :glob:
@@ -140,7 +141,6 @@ Topics in this section will help you get started with ExecuTorch.
    :hidden:
 
    runtime-overview
-   runtime-build-and-cross-compilation
    runtime-backend-delegate-implementation-and-linking
    runtime-custom-memory-allocator
    runtime-error-handling