Clarify public portal changes - #143

Summary:

• keep public /changes release notes focused on portal-facing functionality
• list /changes in the public portal route allowlist table
• include /changes in the conceptual nginx public-route example

Validation:

• .venv/bin/python -m pytest result_server/tests/test_home_template.py result_server/tests/test_portal_version.py result_server/tests/test_portal_access_policy.py
• python3 scripts/tests/check_text_integrity.py
• python3 scripts/tests/check_commit_messages.py
• git diff --check