docs: document sotto setup for a second device

[LouisDeconinck]

Closes #145

Summary

Document the login, setup, and pull sequence for adding a second device, including the Emergency Kit requirement. Keep translated READMEs structurally in sync.

Checks

• scripts/check-readme-i18n
• git diff --check

All commits are signed off under the Developer Certificate of Origin.

[github-actions]

Thanks for your first pull request to Sotto, and welcome.

Two things that trip up newcomers:

• Every commit needs a Signed-off-by line under the Developer Certificate of Origin - PRs with unsigned commits cannot be merged. Sign off with git commit -s; if your branch already has unsigned commits, run git rebase --signoff main and push again.
• Please run cargo fmt --all --check and cargo clippy --workspace --all-targets -- -D warnings before pushing, and add tests for new behaviour (cargo test --workspace runs the suite).

The full guide is in CONTRIBUTING.md - a maintainer will review shortly.