Move program_file_format.md into the new docs tree (#1081)

Summary:

Move program_file_format.md into the new docs tree. Also update the one reference to it.

Reviewed By: mergennachin, JacobSzwejbka

Differential Revision: D50609710

Author: dbort

docs/source/index.rst

@@ -148,6 +148,7 @@ Topics in this section will help you get started with ExecuTorch.
    runtime-backend-delegate-implementation-and-linking
    runtime-platform-abstraction-layer
    portable-cpp-programming
+   pte-file-format
 
 .. toctree::
    :glob:

docs/website/docs/contributors/program_file_format.md renamed to docs/source/pte-file-format.md

@@ -1,12 +1,13 @@
-# Program file format
+# `.pte` file format
 
-ExecuTorch programs are serialized as modified binary flatbuffer files.
+ExecuTorch `.pte` program files are serialized as modified binary flatbuffer
+files with optional data segments appended.
 
 ```
              ┌───────────────────────────────────┐
              │Standard flatbuffer header         │
              ├───────────────────────────────────┤
-             │Optional ExecuTorch extended header│
+Optional ──> │ExecuTorch extended header         │
              ├───────────────────────────────────┤
              │Flatbuffer-serialized program data │
              │                                   │

schema/extended_header.h

@@ -17,8 +17,7 @@ namespace executor {
  * An extended, ExecuTorch-specific header that may be embedded in the
  * serialized Program data header.
  *
- * For details see
- * //executorch/docs/website/docs/contributors/program_file_format.md
+ * For details see //executorch/docs/source/pte-file-format.md
  */
 struct ExtendedHeader {
   /**