Suggestions to improve the reactive expression guide

[maximlt]

I made a few suggestions to improve the reactive guide in here #858 (review):

• the guide relies too much on being executed in a notebook, I don't think you can understand what's going on without executing it to be honest. Either it needs to be updated or we need to allow users to execute the code (e.g. nbsite-pyodide directive, jupyterlite deployment, link to download the notebook).
• if we want this API to feel very generic, I don't think we should start with a DataFrame example, for instance some simple string manipulation should do
• explain that reactive expressions are by default not-reactive, and that's in line with the default behavior of param.depends and param.bind, they only become reactive when a framework supports them, and that Param happens to provide a built-in support for notebooks to automatically refresh the output of a reactive expression.
• explain that setting a reactive expression with .rx.value = <> can only be done on the root
• explain what happens when a reactive expressions has multiple "inputs", e.g. does it cache a re-use the value from a branch that is not updated?