docs: add openedx-proposals links to developer pages

Add a link to the openedx-proposals index on the developers home page
References card and at the top of the references index, with a brief
description of what OEPs are.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: 2 people

source/developers/index.rst

@@ -68,6 +68,7 @@ Open edX Developers
 
       * :doc:`references/running_pr_tests`
       * :doc:`references/developer_guide/index`
+      * :doc:`openedx-proposals:index`
       * :doc:`edx-platform:index`
       * `frontend-platform <https://openedx.github.io/frontend-platform>`_
       * :doc:`references/aspects_home`

source/developers/references/index.rst

@@ -1,6 +1,7 @@
 References Home
 ###############
 
+:doc:`openedx-proposals:index` — Open edX Enhancement Proposals (OEPs) are design documents that describe accepted changes to the Open edX platform. They cover architecture decisions, best practices, and processes that apply across the project.
 
 Per Repo Documentation
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