pw_thread_freertos: add FreeRTOS backends pw::this_thread

Adds id, yield, and sleep backends for pw::this_thread for use on
FreeRTOS.

Change-Id: I374f671880f060d195868fd05e94b0e592b0faad
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/30521
Commit-Queue: Ewout van Bekkum <ewout@google.com>
Pigweed-Auto-Submit: Ewout van Bekkum <ewout@google.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
diff --git a/pw_thread_freertos/docs.rst b/pw_thread_freertos/docs.rst
new file mode 100644
index 0000000..6628718
--- /dev/null
+++ b/pw_thread_freertos/docs.rst
@@ -0,0 +1,8 @@
+.. _module-pw_thread_freertos:
+
+------------------
+pw_thread_freertos
+------------------
+This is a set of backends for pw_thread based on FreeRTOS. It is not ready for
+use, and is under construction.
+