docs: document .dockerignore

[bghira]

Closes #209

Summary

• explain that Cog uses the project directory as its Docker build context
• show how to exclude development-only files with .dockerignore
• document the build-speed and image-size benefits, plus files that should not be excluded
• regenerate docs/llms.txt

Validation

• npx --yes prettier@3.6.2 --check docs/getting-started.md docs/llms.txt
• git diff --check