Add explicit documentation for re-exported platform symbols

Added platform-api.rst to document SimPlatform and attach_data which are
re-exported from submodules but weren't appearing in the main autoapi
index due to sphinx-autoapi limitations with imported members.

This provides users with complete documentation coverage of the public
platform API.

Author: robtaylor

docs/index.rst

@@ -13,3 +13,4 @@ It is developed at https://github.com/chipFlow/chipflow-lib/ and licensed `BSD 2
    chipflow-toml-guide
    chipflow-commands
    API Reference <autoapi/index>
+   platform-api

docs/platform-api.rst

@@ -0,0 +1,17 @@
+Platform API Reference
+======================
+
+This page documents the complete public API of the ``chipflow_lib.platform`` module.
+
+For auto-generated documentation, see :doc:`autoapi/chipflow_lib/platform/index`.
+
+Re-exported Symbols
+-------------------
+
+The following symbols are re-exported from submodules for convenience:
+
+.. autoclass:: chipflow_lib.platform.sim.SimPlatform
+   :members:
+   :undoc-members:
+
+.. autofunction:: chipflow_lib.platform.io.signatures.attach_data