Add Documentation for trt_llm.build configuration

[joostinyi]

:rocket: What

• Adds docs for trt_llm.build key as we open this flow up to external users

  :computer: How

:microscope: Testing

[linear[bot]]

BT-11263 Missing documentation for engine builder usage

[marius-baseten]

All the docs moved to https://github.com/basetenlabs/docs.baseten.co