Update Contribution Page for v18

[shawnweeks]

Describe the bug

Currently the contribution page https://primeng.org/contribution is completely wrong for v18 and the instructions don't work. v18 requires pnpm instead of npm and you can't just run ng serve in the root directory of the project you've got to switch to the apps/showcase directory.

Here is what I had to do instead to run the v18 showcase.

git clone https://github.com/primefaces/primeng.git
cd primeng

npm run setup
cd apps/showcase
ng serve

Environment

N/A

Reproducer

No response

Angular version

18.x.x

PrimeNG version

18.0.0

Build / Runtime

Angular CLI App

Language

TypeScript

Node version (for AoT issues node --version)

v20.15.1

Browser(s)

No response

Steps to reproduce the behavior

No response

Expected behavior

No response