docs: correct suggestion

[emig]

The documentation indicates 2 ways of using CriticalUp's installed Ferrocene:

CriticalUp creates a number of binary proxies which can be used to run the appropriate Ferrocene binaries for a given workspace. These can be added to your shell path on any OS, or used as a rustup toolchain.

But there is no clear suggestion based on the presence/absence of a rust installation:

• rust installed via rustup
• system rust
• no rust

Update to document:
if you have rust installed via rustup, you should link criticalup's ferrocene
if you have rust installed other way than via rustup you should use criticalup run --strict
if you don't have rust installed add binary proxy to the PATH or use criticalup run --strict