[Documentation] Add clearer installation instructions for each architecture

[Cardosaum]

Objective

Users should be able to install RISC Zero's toolchain without any doubts around:

1. If their computer architecture is supported
2. Which command they should use to install RISC Zero's toolchain
3. How to check if the installation succeeded

Obstacle

We've noticed users from architectures other than aarch64-apple-darwin and x86_64-unknown-linux-gnu to have doubts on exact steps on how to install the toolchain and whether their computer architecture would be supported at all.

For x86_64-apple-darwin for example, there is a difference between what we mention in cargo-risczero docs and in our docs website - This generates confusion and uncertainty to if it would be possible to run the r0vm in this architecture at all. (This issue is being tracked at #1641 )

Request

We should provide clear and distinct installation instructions for each path of installing RISC Zero's toolchain.
We should have a subsection in our installation docs for:

• Architectures with prebuilt binaries
• Architectures that have custom installation instructions - (i.e x86_64-apple-darwin)
• How to install and update from source to all other architectures

[linear]

ZKVM-401 [Documentation] Add clearer installation instructions for each architecture