Migrate runtime-platform-abstraction-layer.md (#623)

Summary:
Pull Request resolved: #623

Copy the existing PAL docs, with some minor tweaks.

Reviewed By: digantdesai

Differential Revision: D49932047

fbshipit-source-id: 9c44042d06a2d4784fae2aef7a9935cfbfbfdc76

Author: dbort

docs/source/runtime-platform-abstraction-layer.md

@@ -1,3 +1,36 @@
-# Platform Abstraction Layer
+# Runtime Platform Abstraction Layer (PAL)
 
-TBA
+The ExecuTorch _Platform Abstraction Layer_ (PAL) provides a way for execution
+environments to override operations like:
+- Getting the current time.
+- Printing a log statement.
+- Panicking the process/system.
+
+The PAL function declarations are in
+[`executorch/runtime/platform/platform.h`](https://github.com/pytorch/executorch/blob/main/runtime/platform/platform.h).
+
+## Overriding the default PAL
+
+The default PAL implementation is in
+[`executorch/runtime/platform/target/Posix.cpp`](https://github.com/pytorch/executorch/blob/main/runtime/platform/target/Posix.cpp).
+It uses `std::chrono::steady_clock` for the time, prints log messages to
+`stderr`, and makes other default assumptions.
+
+But, if they don't work for your system, you can override the default PAL by:
+- Including
+  [`executorch/runtime/platform/platform.h`](https://github.com/pytorch/executorch/blob/main/runtime/platform/platform.h)
+  in one of your application's `.c` or `.cpp` files.
+- Defining an implementation of one or more of the `et_pal_*()` functions.
+
+No build system changes necessary. The default PAL functions are weak symbols,
+so providing your own strong-symbol definition will override them at link time.
+
+## Minimal PAL
+
+If you run into build problems because your system doesn't support the functions
+called by `Posix.cpp`, you can instead use the no-op minimal PAL at
+[`executorch/runtime/platform/target/Minimal.cpp`](https://github.com/pytorch/executorch/blob/main/runtime/platform/target/Minimal.cpp)
+by building with the `buck2` flag `-c executorch.pal_default=minimal`. This will
+avoid calling `fprintf()`, `std::chrono::steady_clock`, and anything else that
+`Posix.cpp` uses. But since the `Minimal.cpp` `et_pal_*()` functions are no-ops,
+you will need to override all of them.

docs/website/docs/tutorials/pal.md

@@ -1,4 +1,7 @@
 # Platform Abstraction Layer
+
+DEPRECATED: This document is moving to //executorch/docs/source/runtime-platform-abstraction-layer.md
+
 The ExecuTorch Platform Abstraction Layer (PAL) provides a way for execution
 environments to override operations like:
 - Getting the current time