[mkdocs] docs: add accounts and cryptography concepts

[zero4862]

Adapts Cryptography and Accounts concepts to NEM technology.

Differences with Symbol:

• Hash: NEM uses Keccak-256 / Keccak-512; Symbol uses SHA-3 and SHA-512.
• Signatures: Both use Ed25519, but NEM signs over Keccak-512 while Symbol signs over SHA-512.
• Addresses: NEM addresses are 25 raw bytes / 40 Base32 characters with a 4-byte checksum; Symbol addresses are 24 bytes / 39 characters with a 3-byte checksum.
• Multisig: NEM allows at most 32 cosignatories and is one layer deep; Symbol allows up to 25 cosignatories but supports nested multisig (up to 3 layers).

[zero4862]

@segfaultxavi Ready for review

[segfaultxavi]

👍