[mkdocs] docs: add mosaics concept

[zero4862]

Adapts Mosaics concept to NEM technology.

Differences with Symbol:

• Mosaic ID: <namespace>:<mosaic name> form on NEM (e.g. nem:xem); numeric ID on Symbol (e.g. 0x6BED913FA20223F8), optionally aliased by a namespace.
• Namespace coupling: Every mosaic lives under a namespace on NEM and inherits its parent's lifecycle; mosaics on Symbol exist independently of namespaces.
• Lifecycle: NEM mosaics expire when their parent namespace expires (no separate duration); Symbol mosaics have their own duration (eternal or up to 10 years).
• Properties: Four on NEM (divisibility, initialSupply, supplyMutable, transferable); six on Symbol (adds restrictable and revocable).
• Levy: Not supported on Symbol.
• Description: NEM mosaics carry a free-text description field; Symbol uses the generic metadata instead.
• Creation fee: Fixed at 10 XEM on NEM; variable on Symbol.

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Minor comments

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