[codex] Improve agent onboarding docs

[arjxnt]

Summary

This PR makes the Jolt onboarding path for humans and AI coding agents match the current jolt new scaffold and adds CI coverage so it stays that way.

Changes:

• update the Quickstart guest and host snippets to mirror the generated scaffold in src/main.rs
• update the Hosts guide for the current preprocess_verifier_*(_, _, blindfold_setup) API
• add a new Jolt Book page for AI-agent-oriented proof scaffolding, signature adaptation, input privacy choices, and debugging
• link the new page from the README and book summary
• add scripts/ci/check_docs_scaffold.py to check quickstart/scaffold drift, SUMMARY links, README linkage, and expected agent-workflow sections
• add a Docs workflow that runs the scaffold check and builds the mdBook on docs-related PRs
• document the local docs checks in CONTRIBUTING

Why

The book examples had drifted from what jolt new currently emits. In particular, the quickstart omitted preprocess_shared_*().unwrap(), omitted the third verifier preprocessing argument, and placed #![no_main] in the library snippet even though the generated scaffold keeps that in guest/src/main.rs.

That is exactly the first-run friction new contributors and coding agents hit. The new checker makes that drift visible in CI without requiring a full Rust workspace compile.

Validation

Local:

• python -m py_compile scripts/ci/check_docs_scaffold.py passed
• python scripts/ci/check_docs_scaffold.py passed using the Codex bundled Python runtime
• git diff --check HEAD passed

GitHub:

• Spec Workflow checks passed
• Socket checks passed
• Docs and Build and Test Jolt were enqueued but are currently action_required before any jobs start, which appears to require maintainer approval for forked PR Actions/new workflow execution

Not run locally:

• mdbook build ./book; this Windows environment does not have a working Rust linker/binutils setup for installing mdbook plugins locally
• Rust package tests; MSVC failed due missing link.exe, and GNU failed due missing dlltool.exe on this machine