doc: standardize the naming of accounts

What

This MR proposes to make the naming of accounts more consistent and intuitive, throughout the Octez documentation.

Why

Currently in the doc, the two kinds of Tezos accounts are named quite inconsistently:

• non-programmable accounts are called: implicit accounts = implicit contracts = user accounts
• programmable accounts are called: originated accounts = smart contracts = originated contracts

In the above uses, the following have been reported as particularly confusing:

• Using the term "(implicit) contract" for implicit accounts
• Using the term "originated account" for smart contracts

How

The solution proposed is to:

• remove the confusing terms
• introduce the other acceptable terms as: one preferred term + one synonym
• rewrite mentions everywhere to use the preferred term

There are separate commits for fixing the various definitions and for rewriting their mentions. In particular:

• "smart contract" is chosen as the preferred term for programmable accounts. This seems very reasonable, given the common usage in Tezos and anywhere else.
• "user account" is proposed to be preferred over "implicit account". This may be more debatable, as the latter is widely used in Tezos, but consider that for newcomers (our main target these days):
  • "user account" sounds more intuitive than "implicit account", which is a somewhat opaque and seems to be a idiosyncrasy of Tezos and Near mostly
  • "user account" sounds familiar for people using mainstream blockchains such as Ethereum

The preferred terms are also aligned with those recently adopted in the global documentation <https://docs.tezos.com/architecture/accounts>.

Manually testing the MR

Check the changes and estimate if the new uses are correct, intuitive and consistent.

Checklist

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