Document shell completion setup

[roli-lpci]

Summary

• document how to enable dotenv command completion for Bash, Zsh, Fish, and PowerShell
• identify each shell's startup file and link to Click's caching alternative

Closes #353.

Validation

• generated non-empty completion scripts for all four documented shells from this checkout
• uv run --no-project --with-requirements requirements-docs.txt --with-editable . mkdocs build --strict
• git diff --check