docs: add openapi-code-generator

[mnahkies]

adds https://github.com/mnahkies/openapi-code-generator/ / https://openapi-code-generator.nahkies.co.nz/ to the list.

it currently supports generating typescript client sdks based on fetch / axios, and server routing / request+response validation based on koa (choice of zod / joi for runtime validation).

an experimental nextjs server implementation is in the works (https://github.com/mnahkies/openapi-code-generator/pull/152), and my longer term plan is to add other languages such as kotlin to the set of templates.