docs: document rustup "rustc not applicable" fix for Raspberry Pi builds

[antiguru]

Summary

Adds a troubleshooting subsection to the Raspberry Pi build instructions covering the rustup error:

error: process didn't exit successfully: `rustc -vV` (exit status: 1)
--- stderr
error: the 'rustc' binary, normally provided by the 'rustc' component, is not
applicable to the 'stable-armv7-unknown-linux-gnueabihf' toolchain

Why

This is not a Rahmen build failure — it's a rustup toolchain-state problem: the active toolchain has the cross rust-std target installed but no usable host rustc/cargo, so Cargo's rustc -vV probe fails. It typically follows rustup target add armv7-unknown-linux-gnueabihf (which adds only the cross std, not host tools) or a default-host mismatch.

The new section explains the cause and gives the repair: inspect with rustup show, then reinstall the toolchain with --profile default so host tools are present, fixing the default host first if needed.

Docs-only change.

https://claude.ai/code/session_01AiiEmGRVEqgDq1suA8wzTn

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