pw_thread_embos: adds pw::this_thread support

Adds thread identification, yield, and sleep support for embOS v4.

Change-Id: I9ed552cf83035b4589740f88283b335194dcdc1a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/36421
Reviewed-by: Ewout van Bekkum <ewout@google.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>
Pigweed-Auto-Submit: Ewout van Bekkum <ewout@google.com>
Commit-Queue: Ewout van Bekkum <ewout@google.com>
diff --git a/pw_thread_embos/docs.rst b/pw_thread_embos/docs.rst
new file mode 100644
index 0000000..cad1908
--- /dev/null
+++ b/pw_thread_embos/docs.rst
@@ -0,0 +1,8 @@
+.. _module-pw_thread_embos:
+
+---------------
+pw_thread_embos
+---------------
+This is a set of backends for pw_thread based on embOS v4. It is not ready for
+use, and is under construction.
+