[mkdocs] docs: add namespace concept

[zero4862]

Adapts Namespace concept to NEM technology.

Differences with Symbol:

• Role: Every mosaic lives under a namespace in NEM; optional alias for an account or mosaic in Symbol.
• Mosaic lifecycle coupling: NEM mosaics become inactive when their parent namespace expires; Symbol mosaics persist independently and only the alias breaks.
• Root name length: 16 chars on NEM vs 64 chars on Symbol. Subnamespace levels are 64 in both.
• Duration: Fixed 525600 blocks (~1 year) on NEM vs 30 days to 5 years on Symbol.
• Renewal window: Restricted to the last 43200 blocks before expiration on NEM; any time during the lease on Symbol.
• Lease fee: Fixed on NEM vs duration-proportional XYM on Symbol.
• Subnamespace cap: NEM has no explicit cap beyond the 3-level depth limit; Symbol caps at 256 subnamespaces per root.
• Metadata: NEM does not not support metadata.

[segfaultxavi]

Minor changes only.

[segfaultxavi]

There's a colon to separate namespaces here, but previously you said it was periods. Which one is it?

[zero4862]

NEM namespaces are not used to name mosaics (like in Symbol), but to organize them. Mosaics have their own name. In nem:xem, nem is the namespace name and xem is the mosaic name, separated by a colon.

Subnamespaces are separated by a dot. Example: company.tokens:gold, where company is the namespace, tokens the subnamespace and gold the mosaic name.

[segfaultxavi]

Got it. I had not idea.
Then definitely you cannot use this example here without explaining what the colon means.
I understand you will probably explain it in the Mosaics page, but you need to provide at least a minimal explanation here and a link to where the colon is fully explained.

Something like:

Suggested change

	The `nem` namespace, in particular, is used by the system to host the native currency `nem:xem`.
	The `nem` namespace, in particular, is used by the system to host the native currency `nem:xem`
	(note that the colon separates the namespace from the [currency name](./mosaics.md#fully-qualified-name)).

[zero4862]

Done!

[segfaultxavi]

Got it. I had not idea.
Then definitely you cannot use this example here without explaining what the colon means.
I understand you will probably explain it in the Mosaics page, but you need to provide at least a minimal explanation here and a link to where the colon is fully explained.

Something like:

Suggested change

	The `nem` namespace, in particular, is used by the system to host the native currency `nem:xem`.
	The `nem` namespace, in particular, is used by the system to host the native currency `nem:xem`
	(note that the colon separates the namespace from the [currency name](./mosaics.md#fully-qualified-name)).

[segfaultxavi]

👍