docs: restructure documentation using Diataxis framework
Reorganise all documentation into four Diataxis categories:
Tutorials (3): quick-start, first-model, deploy-local
How-to guides (14): gpu, model-weights, streaming, file-io, concurrency,
deploy, ci-cd, image-size, debug-builds, training, setup-own-model,
private-registry, notebooks, wsl2
Reference (6): yaml, python, http, cli, training, environment
Explanation (5): how-cog-works, why-containers, prediction-lifecycle,
cuda-compat, image-layers
Update mkdocs.yml with grouped navigation and regenerate llms.txt.
