Add pipx to installation documentation

[simonw]

Add to this page: https://datasette.readthedocs.io/en/stable/installation.html

Here's how to install plugins: https://twitter.com/simonw/status/1257348687979778050

$ datasette plugins
[]

$ pipx inject datasette datasette-json-html            
  injected package datasette-json-html into venv datasette
done! ✨ 🌟 ✨

$ datasette plugins
[
    {
        "name": "datasette-json-html",
        "static": false,
        "templates": false,
        "version": "0.6"
    }
]

[simonw]

pypa/pipx#79 showed me how to upgrade plugins within the pipx virtual environment.

[simonw]

New documentation is here: https://datasette.readthedocs.io/en/latest/installation.html#install-using-pipx