Update 03_3_Setting_Up_Your_Wallet.md to descriptor wallets by default

[willcl-ark]

Question: perhaps it is time to update this section to use descriptor wallets?

The documentation does clearly indicate setup of a legacy wallet however now that descriptor wallets are default, but with legacy wallets essentially feature-frozen, and with some indication that legacy wallets may be deprecated (but supported using a migration tool) in the future, maybe it would be more useful to use them by default.

[kouloumos]

Related: BlockchainCommons/Community#141