feat: list supported standards and documents

[PRAteek-singHWY]

Summary

• add a new /rest/v1/supported_documents API endpoint that returns unique node names grouped by document type
• add a new frontend page at /supported_documents to display supported sources in grouped cards
• make each source name clickable to existing OpenCRE routes (/node/<type>/<name>) for drill-down
• add a Sources navigation link in desktop and mobile headers
• document the endpoint in OpenAPI and add backend tests

Why

Issue #383 asks for a clear list of standards/documents supported by OpenCRE so users and chatbot workflows can discover available sources quickly.

Validation

• ./venv/bin/python -m unittest application.tests.db_test.TestDB.test_supported_documents application.tests.web_main_test.TestMain.test_supported_documents_from_db

• npm run build

##Screenshot

Closes #383