Rename getting-started-build-and-cross-compilation back to runtime-build-and-cross-compilation (#895)

Summary:
Pull Request resolved: #895

There are a bunch of files that are dependent on linking to runtime-build-and-cross-compilation so renaming it back. D50250770 was the original rename.

Reviewed By: dbort

Differential Revision: D50272497

fbshipit-source-id: 8c87dafd61ae36c9cddf2ca2a7f76c9f0dc8020f

Author: tarun292

docs/source/index.rst

@@ -78,7 +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
+   runtime-build-and-cross-compilation
 
 .. toctree::
    :glob: