[mkdocs] docs: add transfer transctions concept

[zero4862]

Adapts Transfer Transaction concept to NEM technology.

Differences with Symbol:

• Recipient: 40 chars (NEM) vs 39 chars + namespace alias (Symbol).
• XEM amount field: Present in NEM with dual transfer-amount/multiplier semantics; absent in Symbol (XYM is just another mosaic).
• Message type field: 16-bit type code on NEM (0x0001 / 0x0002) vs Symbol's single prefix byte (0x00 / 0x01).
• Encryption schemes: NEM tooling has both AES-CBC and AES-GCM (new); Symbol's documented convention is AES-CBC only.
• Aggregation: NEM lacks aggregate transactions.

[segfaultxavi]

Good!