Update local webapp development instructions

[natalyjazzviolin]

Tell us about the documentation you'd like us to add or update.

Local webapp development.
Add nvm install command prior to npm install

If applicable, add links to the relevant docs that should be updated

• Develop on airbyte-webapp

[timroes]

@natalyjazzviolin I'm at the moment working on #22053 which will switch over to use pnpm as a package manager. I'll anyway need to document how to use that in this file, so I'll take this over if you don't mind.