Update docs to use corepack instead of nvm+yvm

[justinjao]

Fix cBioPortal/icebox#531

Describe changes proposed in this pull request:

• This PR updates the setup documentation to utilize Corepack for versioning package managers, as opposed to the previously recommended Node Version Manager (nvm) and Yarn Version Manager (yvm), which is now outdated.

[alisman]

Thanks @justinjao, i'm going to test this out. It's odd that Corepack is experimental but nvm/yvm deprecated already, no?

[justinjao]

Thanks @justinjao, i'm going to test this out. It's odd that Corepack is experimental but nvm/yvm deprecated already, no?

Thank you! Let me know if anything is unclear. And yes, I agree, seems like everyone's moving forward with it, so that "experimental" tag may not be so "experimental" anymore!