docs(onboarding): add provider starter templates

[typelicious]

What changed

• add starter provider snippets for cloud openai-compat, local-worker, and image-provider setups
• link the snippets from onboarding, integrations, and README helper documentation
• extend the docs examples area from client templates to provider templates

Why

• make the first provider rollout path as copy/pasteable as the client onboarding path
• reduce friction for common gateway topologies during v0.8 onboarding work

How verified

• ./.venv-check-313/bin/ruff check README.md docs/INTEGRATIONS.md docs/ONBOARDING.md CHANGELOG.md
• git diff --check
• python3 sanity check for new docs/examples provider files