Move 'Installing from source' contents from rust-lang repo README to forge

[crlf0710]

This is cut-paste from the original README.md.

cc rust-lang/rust#118627

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @pietroalbini (or someone else) soon.

[ehuss]

My personal opinion is that this does not belong in the forge. The forge is mostly geared towards members of the rust-lang org, not users. This also seems like it would significantly increase the risk of it getting out of date, and increase the hassle when a PR changes something that requires updating these instructions. Would it be possible to keep this in rust-lang/rust in a separate file?

[crlf0710]

But the "Other Installation Methods" chapter is already here. Do you feel there's major difference between those? Or is the other chapter better find a new home somewhere? @ehuss

[ehuss]

I agree that the "Other installation methods" page probably needs a better home (since it is the only user-centric page in this book), but where that is isn't clear. I have considered creating a dedicated book on "installing rust", but I also lament that it seems like a failure if installing rust is so complicated it needs its own documentation.

In particular, that documentation is generally not version-specific (in that it doesn't really change much from version to version). However, the documentation proposed in this RFC will be version specific, and I think should be kept in the source tarball that the user downloads, so they can inspect the instructions directly inside it, and so that it is current for the version that they have. In the forge, there would not be a way to easily have different instructions for different versions.