docs: add CONTRIBUTING.md to .github with lifecycle diagram and stand…

[vjuliaife]

Summary

• Creates .github/CONTRIBUTING.md — GitHub's primary contributor guide location (surfaced automatically on new PRs and
  issues)
• Adds an end-to-end Mermaid data lifecycle diagram tracing the full event journey: Stellar Network → Soroban RPC
  (network layer) → Event Indexer (parsing worker) → Translation Engine → PostgreSQL + WebSocket (parallel paths) →
  Frontend Dashboard
• Documents 3-step local setup (git clone + npm install → cp .env.example .env.local → npm run dev:ws) satisfying the
  "spin up in 3 steps" acceptance criterion
• Consolidates team style guidelines (function declarations, no any, naming conventions), commit format
  ((): ), PR procedures, and a pre-PR validation checklist in one place
• No existing files modified — root CONTRIBUTING.md and ARCHITECTURE.md are preserved and linked from the new file

What was resolved

• Mermaid diagram uses flowchart TB syntax consistent with the existing ARCHITECTURE.md, which renders correctly on
  GitHub's standard markdown interface
• PostgreSQL is correctly shown as a parallel persistence branch (via app/api/ routes), not in the real-time WebSocket
  streaming pipeline
• .env.example was confirmed to exist, so Step 2 works out of the box

Closes #102

[Osuochasam]

@vjuliaife switch branch, your pushing to my main branch

[vjuliaife]

@vjuliaife switch branch, your pushing to my main branch

There is no dev branch or any other branch that looks better to push to, i had to push to main. By the way, you have been merging other works on main branch.

Please create a branch i can create the pr to. I had to create another pr to this branch #210

let me know which works for you