Add the spec of using bip44 path into docs

[crackerli]

In the future we will see many wallets app, mobile, desktop, web. I think most of them will implemented as a HD wallet. For bip44, Namada need to restrict the usage of each field and give out a spec, then mnemonic words generated on various wallets can work anywhere.

As of now, the docs https://docs.namada.net/users/transparent-accounts only give the default path description, but did not describe how to derive other keys.

For exmaple:
To derive the second address, use m/44'/877'/0'/0'/1' or m/44'/877'/1'/0'/0', or other more formats are valid.

BRs

[cwgoes]

@bengtlofgren can you check this / transfer the issue if still applicable?