[mkdocs] docs: add transactions concept

[zero4862]

Adapts Transactions concept to NEM technology.

Differences with Symbol:

• Multisig transactions: NEM wraps a single inner transaction per multisig envelope; Symbol aggregates wrap multiple inner transactions at once.
• Multisig fees: NEM bills each transaction in a multisig composition separately (inner + envelope + consignatures), all paid by the multisig account; Symbol charges a single fee for the aggregate.
• Transaction fees: NEM's fee field is the exact amount paid; Symbol's is a maximum, with the actual charge determined by the block's fee multiplier.
• Block cadence: NEM targets a 60-second block time; Symbol targets 30 seconds.
• Irreversibility: NEM considers a transaction irreversible after 360 confirmations (~6 hours) via the block-chain rewrite limit; Symbol relies on explicit finalization rounds.
• Transaction catalog: NEM offer less transactions types than Symbol.

[segfaultxavi]

Good!