feat: reorganize examples for upcoming namespace support

[moul]

• move examples/gno.land/* to future namespaces r/system, ...
• update contracts’ imports.
• update genesis with new paths.
• update various links pointing to previous URLs.

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For the naming, I suggest:

• r/sys/{ns,gnode,...} -> the contributor-dedicated realms that manages most of the chain.
• r/sys/ns -> the contributor-dedicated r/users alternative that will be used to filter package names by prefixes.
• r/sys/gnode -> the top-level gnode.
• r/gnoland/{boards,users,gnode,...} -> the community official realms.
• r/gnolang/{gnode,...} -> the gnolang gnode and future gnolang-dedicated realms, I any.

Depends on #365
Blocks #375

[moul]

Update, after the first discussion with Jae

• p/demo/gdao

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• r/demo/boards
• r/demo/nft
• r/demo/foo20
• r/demo/users
• r/gnoland/dao
• r/system/names
• r/system/validators