Rephrase "Share" to be "Download"

[edonehoo]

To better support #559, can we change "Share" terminology to "Download" across examples/demos? And use the fa-download icon instead? Feel free to message me about this!

This phrasing better aligns with the action of downloading a chat transcript file, since we're not necessarily planning to generate a shared link by default.

Need the same updates in design guidelines -- can use this same issue or open a separate one.

[rebeccaalpert]

Can definitely update demos - I think we likely want to keep the "share" icon in the code as an option so it's not breaking, but we don't need to show it necessarily.