pw_sync_baremetal: Add scaffolding for baremetal

Adds a dummy spin lock backend implementation for baremetal
configuration.

The provided implementation makes a single attempt to acquire the
lock and asserts if it is unavailable. It does not perform interrupt
masking or disable global interrupts, so this implementation does
not support simultaneous multi-threaded environments including IRQs,
and is only meant to prevent data corruption.

Change-Id: I4d44b2dd5cc6f38be1188a9f480f61c6d042b855
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/30520
Reviewed-by: Ewout van Bekkum <ewout@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Prashanth Swaminathan <prashanthsw@google.com>
diff --git a/pw_sync_baremetal/docs.rst b/pw_sync_baremetal/docs.rst
new file mode 100644
index 0000000..1fbd99e
--- /dev/null
+++ b/pw_sync_baremetal/docs.rst
@@ -0,0 +1,11 @@
+.. _module-pw_sync_baremetal:
+
+-----------------
+pw_sync_baremetal
+-----------------
+This is a set of backends for pw_sync that works on baremetal targets. It is not
+ready for use, and is under construction. The provided implementation makes a
+single attempt to acquire the lock and asserts if it is unavailable. It does not
+perform interrupt masking or disable global interrupts, so this implementation
+does not support simultaneous multi-threaded environments including IRQs, and is
+only meant to prevent data corruption.