Make sure all docs and example derivation paths correspond to the network

[notmandatory]

          unrelated to this PR, but the derivation path should correspond to the network, e.g. `m/86h/1h/0h/0`

Originally posted by @ValuedMammal in #1028 (comment)

[ValuedMammal]

This is the only place I noticed where Network::Testnet is used but the derivation path starts with m/86h/0h. I guess nothing prevents you from using any deriv path you like (the network is checked against the xpub?), but for consistency and as a general best practice, testnet examples should also use the testnet coin type as described in BIP44, for example m/86h/1h.

bdk/crates/bdk/examples/mnemonic_to_descriptors.rs

Lines 35 to 37 in 46d39be

	// define external and internal derivation key path
	let external_path = DerivationPath::from_str("m/86h/0h/0h/0").unwrap();
	let internal_path = DerivationPath::from_str("m/86h/0h/0h/1").unwrap();

If others agree, this could probably get lumped in with #1223

[notmandatory]

Thanks for checking @ValuedMammal, @realeinherjar can you fix this in your #1223?

[ValuedMammal]

From what I can tell, all of our documented examples use a derivation path that makes sense for the network, in particular using coin type 1 for testnet. Also addressed by #1286, #1378