docs: add deploy-your-own-splitter guide

[olaleyeolajide81-sketch]

Summary

Closes #143. Adds a step-by-step guide for building and deploying your own splitter contract to the Stellar testnet using the Stellar CLI.

• New docs/deploy.md: prerequisites, building the wasm32v1-none wasm, deploying via scripts/deploy.sh, smoke-testing with scripts/demo.sh, regenerating the TypeScript SDK bindings from the deployed instance, optionally wiring up the web dashboard, and a troubleshooting section.
• References scripts/deploy.sh throughout (prerequisites, deploy step, troubleshooting).
• Linked from README.md.
• Added docs/deploy.md to the CI markdown-link-check job so broken doc links fail CI.

Why

Issue #143 asked for a deploy-your-own-splitter guide that walks through building and deploying the contract with the Stellar CLI and references scripts/deploy.sh. This gives contributors and fork maintainers a clear, copy-pasteable path from source checkout to a live contract instance and a regenerated SDK.

Acceptance Criteria

• Guide references scripts/deploy.sh (prerequisites, step 2, troubleshooting).
• One concern per PR — this is a documentation-only change.
• No behavior change, so no contract tests are required; contract code is untouched.
• no_std / typed-error contract constraints are unaffected (no contract code changes).
• CI gate updated: markdown-link-check now includes docs/deploy.md and passed locally.

Verification

• markdown-link-check on README.md + all docs/*.md (including deploy.md): passed locally (exit 0, no broken links).
• cargo fmt --all, cargo clippy --all-targets -- -D warnings, and cargo test are unaffected by a docs-only change; npm run build for the sdk/app is unaffected.

Test plan

1. cargo build --release --target wasm32v1-none -p tributary-splitter
2. stellar keys generate deployer --network testnet --fund
3. ./scripts/deploy.sh deployer
4. ./scripts/demo.sh deployer
5. stellar contract bindings typescript --contract-id "$(stellar contract id splitter --network testnet)" --network testnet --output-dir sdk --overwrite

Checklist

• Linked issue (Add a deploy-your-own-splitter guide #143)
• cargo fmt --all / cargo clippy --all-targets -- -D warnings pass (no Rust changes)
• cargo test passes (no Rust changes)
• No new behavior requiring tests

closes #143

[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.