docs(dev): write local development setup guide

[Demilade10]

Description

Write a step-by-step local development setup guide so new contributors can get from clone to a passing test suite without needing to ask a maintainer for basic setup help.

Requirements and Context

• Setup knowledge (Rust/Cargo versions, SDK pin, wasm target, Windows build quirks) currently only exists in maintainer context and is not documented anywhere in the repo
• Must explain the soroban-sdk = 27.0.3 pin specifically (27.0.5 is missing soroban-ledger-snapshot on crates.io, so it cannot be used)
• Must explain the wasm32v1-none build target requirement
• Must explain the crate-type distinction: ["rlib"] is required for cargo test to pass, ["cdylib", "rlib"] is required for stellar contract build to succeed - this is a Windows/MinGW constraint contributors will hit if not documented

Suggested Execution

Branch name: docs/dev-setup-guide

Files to touch:

• docs/DEVELOPMENT.md (new)
• README.md (add link to new doc)
• CONTRIBUTING.md (add link to new doc)

Example commit message: docs(dev): add local development setup guide

Test and Commit

• docs/DEVELOPMENT.md created covering Rust/Cargo version requirements and Stellar CLI install + version
• Explains the soroban-sdk 27.0.3 pin and why 27.0.5 cannot be used
• Explains wasm32v1-none build target setup
• Explains the crate-type distinction and why both configurations are needed
• Includes exact, copy-pasteable commands to run tests locally
• Includes exact, copy-pasteable commands to build and deploy to testnet
• Doc is linked from both README.md and CONTRIBUTING.md
• A contributor unfamiliar with the repo should be able to follow it start to finish without errors

Guidelines

• Assignment required before starting - comment to request assignment
• PR description must include: Closes docs(dev): write local development setup guide #3

Complexity

Trivial - 100 points

[abhiramvsmg]

Hi @Demilade10, I'd like to work on this — could you assign it to me? I'll follow the suggested branch name and file structure from the issue.