Add an SDK quickstart page (#134)

[olaleyeolajide81-sketch]

Summary

Closes #134. New users needed a short path from install to first call. This adds docs/quickstart.md, a five-minute walkthrough of the TypeScript SDK, and links it from the README.

What the quickstart covers

• Prerequisites (Node, funded testnet account, a signer)
• Install via npm (and build from repo checkout)
• Read-only call: get_split
• First write: create_split with a minimal inline signer
• Pay through the split: preview_payout then pay
• Pointers to the SDK README, architecture, glossary, and main README

Acceptance criteria

• Quickstart doc (docs/quickstart.md)
• README link to the quickstart
• CI gate updated — docs/quickstart.md added to the markdown-link-check step in .github/workflows/ci.yml

Verification

• npm run build (sdk, tsc) — passes
• markdown-link-check on README.md, docs/architecture.md, docs/integrations.md, docs/mainnet.md, and docs/quickstart.md — passes (no broken links)
• Rust gates (cargo fmt --all --check, cargo clippy --all-targets -- -D warnings, cargo test) were not run in this environment (no Rust toolchain), but this change is documentation-only; no contract or SDK code behavior changed, so no_std/typed-error rules and Rust tests are unaffected.

Test plan

• Confirm the quickstart snippets read correctly against the live testnet deployment
• Confirm CI passes (markdown links, sdk build, contract fmt/clippy/test)

closes #134

[Spagero763]

This PR's contract check is failing on markdown-link-check / cspell, but not because of anything in your diff — your branch was forked before a docs fix landed on main (the flagged word/link doesn't exist in your changes, and main passes this check cleanly). Since "Update branch" is enabled on this repo, click it on this PR and CI should go green without any code changes on your end.