docs: add contributor quickstart guide

[AryanXCode646]

Summary

Adds a concise contributor quickstart guide (docs/CONTRIBUTOR_QUICKSTART.md) to make it easier for new contributors to set up OnionScan and validate changes locally.

Problem

New contributors previously had to piece together local development prerequisites, SQLite/CGO setup, build commands, and testing workflows across multiple architectural and governance documents.

Solution

Created a dedicated, practical docs/CONTRIBUTOR_QUICKSTART.md guide covering prerequisites, repository configuration, CLI/daemon build instructions, test commands, contribution workflow, troubleshooting, and defensive-use reminders.

Changes

• Added docs/CONTRIBUTOR_QUICKSTART.md with step-by-step developer setup and validation guidance.

Testing

• Formatting validated with test -z "$(gofmt -l .)" and git diff --check
• Go static analysis executed (go vet ./...)
• Local build and test suite executed (go test -v -race ./...)
• Confirmed no unrelated source files were modified

Collaboration

This contribution was prepared collaboratively by @AryanXCode646 and @ashishsinghbora.