docs: Fix outdated setup instructions in CONTRIBUTING.md

[te-ing]

---

name: Pull Request
about: Create a pull request to improve this repository
title: Fix outdated setup instructions in CONTRIBUTING.md
labels: "docs"
assignees: te-ing

Description

I’ve been using react-datepicker for a long time and recently decided to start contributing to the project.

While preparing to get involved, I noticed that parts of the CONTRIBUTING document were slightly outdated or unclear.

This PR updates and clarifies those sections to help new contributors onboard more smoothly.

Problem

yarn link is no longer needed.

Changes

• yarn link steps (unnecessary - docs-site uses portal: dependency)
• Setup flow reduced from 9 steps to 7 steps
• Moved yarn install to step 3 (before build steps)
• Updated testing section for Jest & React Testing Library

Contribution checklist

• I have followed the contributing guidelines.
• I have added sufficient test coverage for my changes.
• I have formatted my code with Prettier and checked for linting issues with ESLint for code readability.