docs: add OS abstraction layers doc

Adds a new top level doc on Pigweed's OS abstraction layers.
While doing so, also adds some minor updates to the relevant modules.

Change-Id: Ie5c324088d06a1b4c6c11ff5abbbea03f4e9c61d
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/44920
Commit-Queue: Ewout van Bekkum <ewout@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
diff --git a/docs/index.rst b/docs/index.rst
index 8d996e4..28cf4ab 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -15,6 +15,7 @@
   docs/code_of_conduct
   docs/embedded_cpp_guide
   Code Style <docs/style_guide>
+  docs/os_abstraction_layers
   targets
   Build System <build_system>
   FAQ <docs/faq>