pw_thread_embos: add thread creation for embOS

Adds thread creation support to embOS.

Also updates existing documentation to reflect that this is now
supported.

Change-Id: Id95dbb5f7b283733b7cd071831ddcf95c0c549d0
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/44144
Reviewed-by: Keir Mierle <keir@google.com>
Commit-Queue: Ewout van Bekkum <ewout@google.com>
Pigweed-Auto-Submit: Ewout van Bekkum <ewout@google.com>
diff --git a/docs/module_structure.rst b/docs/module_structure.rst
index 17d3a07..dc2e4b0 100644
--- a/docs/module_structure.rst
+++ b/docs/module_structure.rst
@@ -188,6 +188,8 @@
     BUILD.gn
     README.md
 
+.. _module-structure-compile-time-configuration:
+
 Compile-time configuration
 ~~~~~~~~~~~~~~~~~~~~~~~~~~
 Pigweed modules are intended to be used in a wide variety of environments.