docs: document shell completions

[LouisDeconinck]

Summary

Document sotto completions <shell> with Bash, Zsh, and Fish examples, and note that release archives bundle the completion files.

Checks

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

Closes #133

[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.

[Maxerns]

One optional nit for a future touch-up: the zsh parenthetical ("add the directory to fpath
…") is still in English inside the translated READMEs. But otherwise its fine