docs: progressive learning structure with per-executor guides and architecture reference

[ko3n1g]

Summary

• Adds quickstart.md — a 5-minute working example using run.Script + LocalExecutor, no cluster or SSH required
• Adds executors/ directory with seven per-executor guides (local, docker, slurm, skypilot, dgxcloud, lepton, kuberay), each following a consistent structure: prerequisites → annotated config → e2e workflow → advanced options
• Adds architecture.md — internals reference covering the Experiment call chain, Executor→TorchX scheduler mapping, metadata storage layout, and contributor steps for adding a new executor
• Refactors execution.md — removes the per-executor sections (now in executors/); keeps packager/launcher reference; adds links to executors/ and architecture.md
• Extends management.md — adds a "Putting it all together" e2e section at the end that ties config + remote executor + Experiment.from_id together
• Improves ray.md — adds a "When to use Ray vs. standard execution" decision table, per-backend prerequisites, and cross-links to the relevant executor guide before each quick-start section; marks CustomJobDetails as an advanced pattern
• Reorders index.md toctree to match the progressive learning path: quickstart → configuration → execution → executors/ → management → cli → ray → architecture

Guide ordering rationale

Each level deliberately avoids introducing concepts from the next:

• quickstart — LocalExecutor only, no packager/SSH
• executors/ — ordered by setup cost (local → docker → slurm → skypilot → dgxcloud → lepton → kuberay)
• management "putting it all together" references a remote executor so it lands after executors/
• architecture is last — internals make sense only once the user-facing model is understood

Test plan

• sphinx-build docs/ _build/html — all toctree references resolve with no warnings
• Walk each new e2e snippet to confirm it is syntactically correct and runnable
• Check cross-links between executors/, ray.md, and architecture.md are consistent

🤖 Generated with Claude Code