docs(nx-object): repoint the dead format references

[LNSD]

The README and AGENTS.md both still described the layers by a feature that no longer exists, and a third of the wiki pages the crate cites have been renamed or removed since they were written, so a reader following one lands on a 404 rather than on the structure the code mirrors.

• Rewrite the README against the current runtime tiers: raw and read need no allocator, write needs a heap, and only the from_directory builders need a filesystem. Drop the feature table, which restated what each feature's own doc comment already says.
• Repoint every reference that moved: KIP to KIP1, the MOD0 header to MOD, and PFS0 to the NCA page that absorbed it, along with the section anchors switchbrew renamed on the NRO, NSO, NACP and NPDM pages.
• Cite the 3DS page for RomFS, whose layout the console inherits unchanged, since switchbrew documents the format nowhere.
• Name the layers in the crate docs rather than linking them, so the table can describe the crate whole without a lean build leaving a link with nothing to resolve to.
• Replace AGENTS.md's filesystem-support feature table with the two axes that select a build now, correct the no_std proof to the command CI runs, and point at the ## doc comments in Cargo.toml as the authoritative description of each feature.