Use Transaction instead of Deploy in the docs

[ipopescu]

Starting with 2.0, we are introducing the term "transaction." All our docs should use this term instead of "deploy."

Sub-tasks:

1. Add a new page/section/note explaining a legacy deploy vs transaction. A deploy is a transaction. If it doesn't work, it is trying to do something that we don't support.

https://github.com/casper-network/casper-node/blob/1934a4f285ced7fb0e526a9b57f0b298ce323619/types/src/transaction.rs#L125

Find/replace all deploys with transactions, but also read the context to make sure it makes sense after the change.

Explain where appropriate that Deploys are deprecated but can be wrapped into a Transaction to be executed after 2.0. They will not be supported in the future, probably in 3.x.

pub enum Transaction { /// A deploy. Deploy(Deploy), /// A version 1 transaction. #[cfg_attr(any(feature = "std", test), serde(rename = "Version1"))] V1(TransactionV1), }

2. Replace all mentions of deploy with Transaction