add epicardium api development stub

en/firmware/epicardium_api_development.md

+---
+title: Epicardium API Development
+---
+
+# Epicardium API Development
+
+## Overview
+
+The Epicardium API allowsd user code running on core 1 (running the pycardium) to access functionality running on the epicardium (core 0).
+
+In other words: It allows you to call C code running on the Epicardium from a Python module.
+
+## API definition
+
+## API implementation
+
+## Build System Integration
+
+## Best Practices
+
+## Internals