Setup instructions refer to non-existent file.

[ttdoucet]

The README.md says to follow the setup instruction here:

https://github.com/chipsalliance/chisel/blob/main/SETUP.md

but the file does not exist. It makes getting started rather difficult.

[jackkoenig]

My apologies, I intended to delete all references to that file in #3764 but I clearly missed at least one. The new setup instructions are on the website: https://www.chisel-lang.org/docs/installation

I'll fix any lingering links to SETUP.md.

[jackkoenig]

The remaining links to SETUP.md have been removed.