docs: add overview, contracts reference, service architecture pages

[jean-neiverth]

Summary

• New: overview.md — "What is BYOS" page with problem statement (solver onboarding barrier with bonding pool numbers), before/after contrast, how BYOS works at a high level, revenue model, consolidated rejection reasons table, and slashing policy summary
• New: contracts.md — Contracts reference with Escrow/Trampoline/TrampolineFactory interfaces (functions, events, errors), how BYOS interacts with each contract, and key design decisions consolidated from contracts ADRs
• New: service.md — Service architecture with two-listener model, async validation loop, proposal lifecycle, scoring, gas cut, persistence model, and key design decisions consolidated from service ADRs
• Enhanced: sub-solver integration guide — Added explicit responsibility framing ("you ARE / you are NOT responsible for"), API endpoint summary table, and TypeScript reference implementation alongside the Rust one
• Updated vitepress sidebar/nav and README entry points

Context

Gap analysis against expected documentation coverage identified these missing areas:

1. No accessible "what is BYOS" overview for newcomers
2. No dedicated contracts or service reference pages (details were embedded in the 72KB design document)
3. Sub-solver guide lacked explicit role framing, API summary, and TS example reference
4. Design decisions from implementation repo ADRs were not surfaced in the shared docs