docs: update pnpm example with caching

[ethanneff]

Description: update the example to not install dependencies twice

Related issue: based on this guide https://github.com/pnpm/action-setup?tab=readme-ov-file#use-cache-to-reduce-installation-time

Check list:

• [x] Mark if documentation changes are required.
• [x] Mark if tests were added or updated to cover the changes.