Add architecture and design principles doc

[kate-osborn]

Proposed changes

Adds architecture and design principles doc.

Problem: The project has no document that explains the architecture or design principles.

Solution: I added a document that describes the high-level architecture and proposes some design principles.

The architecture portions of the doc are high-level. I mimicked the structure and diagrams from the first half of NIC's How NGINX Ingress Controller Works doc, but I did not include any diagrams or explanations of the architecture of the code. I anticipate the code's architecture will change and do not want to commit us to update this doc every time we make a code change. As our architecture stabilizes, we can expand this document to provide a more comprehensive overview.

Testing: None

Please focus on (optional): The design principles are aspirational. We may not follow all of them today. They are based on a document that @pleshakov started early in the project. I am looking for feedback on them; they are all up for debate.

Closes #673

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

[ADubhlaoich]

Made a few suggestions, but otherwise LGTM! Very well written, and easy to understand.