Additional Stacks.js examples

[obycode]

A user on discord ran into an issue understanding how to get a fee estimate and then sign the transaction properly. (Thanks for responding to this one @janniks 🙏) I don't see a good example of this in our docs, and it seems like a common scenario.

Another common question I see that I don't think we have good documentation on is how to wait for the result from a transaction (or save the txid and check back later for result).