[Feature]: Update README install instructions with pipx

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### Description

Related to: https://github.com/linode/linode-cli?tab=readme-ov-file#install

Current installation command:
`pip3 install linode-cli`

Recommended installation command:
`pipx install linode-cli`

Potentially also include upgrade command on README:
`pipx upgrade linode-cli`

Linux distributions such as Fedora and Debian now mark system Python environments as externally managed (PEP 668), which prevents or discourages installing packages globally with pip because doing so can overwrite OS-managed dependencies and potentially break system tools. pipx is designed specifically for installing Python-based CLI applications in isolated virtual environments, allowing users to install, upgrade, and remove tools like linode-cli without modifying the system Python environment. Recommending pipx aligns with current Python packaging best practices, avoids dependency conflicts, and ensures a safer and more predictable installation experience across modern Linux systems.

Thanks for your consideration! And a great CLI tool.

### Potential Usage Example

_No response_

Apri la sezione sull’installazione della README collegata nell’issue e controlla il comando pip3 attuale. Aggiorna le istruzioni per consigliare l’installazione tramite pipx e includi il comando di upgrade proposto, se appropriato; il lavoro è completato quando la README riflette il workflow di installazione più sicuro per linode-cli.

Scritto dal modello di indicizzazione a partire dal testo della issue.