Improve Account Documentation

[robert-zaremba]

Summary

In SDK we have few different accounts: BaseAccount, ModuleAccount (maybe more). We lack details about it in the /docs. Especially related to the structure, and the whole concept.

Problem Definition

Both for SDK developers and the app developers, missing of a good description of an account abstraction can be a blocker for building advanced things related to accounts.

Proposal

Create a spec and documentation for the current concept of accounts.

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[robert-zaremba]

cc: @amaurymartiny , @clevinson

[amaury1093]

One confusion I had when reading docs about accounts was the introduction of account numbers. A couple of reasons for this introduction were given in discord, having them documented would be a good idea.

[julienrbrt]

auth accounts types are being phased out for x/accounts, which has an extensive documentation coming up: #22099.