docs: how to build an Engine

[freelerobot]

• For Devs wanting to add a new inference engine to Cortex/Jan
• Provide a clear example and walk through adding a sample engine
• Specify the cortex Engines abstraction
• How version control & variant works
• Other necessary details 🙏🙏

[dan-menlo]

@gabrielle-ong Can you QA the writeup and see if there are improvements needed?