[Feature]: Update README install instructions with pipx

Nobody has claimed this yet.

### 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_

Open the README installation section linked in the issue and review the current pip3 command. Update the instructions to recommend pipx installation and include the proposed upgrade command if appropriate; done means the README reflects the safer installation workflow for linode-cli.

Written by the indexing model from the issue text.