docs: Add link to in-progress hardware targets

Change-Id: I106e113b94bfde3056abf6dc4bed843d2e5316f5
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/171239
Commit-Queue: Auto-Submit <auto-submit@pigweed-service-accounts.iam.gserviceaccount.com>
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Armando Montanez <amontanez@google.com>
Reviewed-by: Hector Ayala <hayala@google.com>

docs/targets.rst

@@ -104,3 +104,8 @@ The following is a list of targets used for upstream Pigweed development.
   :glob:
 
   targets/*/target_docs
+
+Work-in-progress targets
+------------------------
+You can see a list of hardware targets that are under development but not
+yet landed `here <https://issues.pigweed.dev/issues/300646347/dependencies>`_.