docs: expand CONTRIBUTING guide and add PR template

[olalois]

Summary

Expands the bare CONTRIBUTING.md and adds .github/PULL_REQUEST_TEMPLATE.md to address the recurring new-contributor mistakes identified in the issue (wrong base branch, wrong PR target, missing conventional-commit prefix).

Closes #91

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Type of change

• docs — changes to meta-docs (CONTRIBUTING.md, README.md, templates)

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Branch checklist

• I branched from develop, not main
• This PR targets develop, not main
• My PR title follows conventional commits format

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Content checklist

• No marketing copy — descriptions explain what, not how great
• N/A — no MDX pages modified

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Snippet checklist

• N/A — no .mdx files with code fences were added or modified

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What's included

CONTRIBUTING.md

• Branch model section (develop = trunk, main = release) with visual diagram and prominent warning callout
• Getting started: prerequisites, fork/clone, pnpm install, upstream sync, branch naming convention table
• Making changes: file structure, writing style rules, MDX conventions
• Commit messages: conventional commits reference table, examples, and rules
• Opening a pull request: step-by-step guide + common mistakes table
• Code and snippet standards: how the checker works, no-check usage policy, Stellar testnet job behaviour
• CI checks table (blocking vs non-blocking)
• Review process: squash-merge policy, develop → main release cadence

.github/PULL_REQUEST_TEMPLATE.md

• Summary field with Closes # pre-filled
• Type of change checkboxes (6 conventional commit types)
• Branch checklist targeting the three recurring mistakes from the issue
• Content checklist (TypeScript / Chain enum / no marketing copy)
• Snippet checklist with pnpm run check:snippets reminder and no-check justification prompt

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