docs: 📝 add pnpm to the getting started and local setup guides

[VoloshchenkoAl]

Description

Added pnpm to the getting started and local setup guides in the documentation

Motivation and Context

I hope these changes will help other developers that use pnpm easily set up commitlint

Checklist:

• I have updated the documentation accordingly.
• All new and existing tests passed.

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[escapedcat]

Thanks!