Update from code changes: Cobalt B20 seize surface

[mintlify]

Summary

Documents the first-class administrative seize operation coming to the shared B20 interface at the Cobalt hardfork and marks burnBlocked as the deprecated predecessor.

Changes

• New base-chain/specs/upgrades/cobalt/b20-seize-surface.mdx covering seizeWithMemo, the new SEIZE_ROLE, SEIZE_HOLDER_POLICY / SEIZE_RECEIVER_POLICY scopes, the new Seized event, AccountNotSeizable error, the PausableFeature.SEIZE ordinal, the opt-in-per-token default behavior, and a migration path from burnBlocked to seize-then-burn.
• Updated the Beryl B20 reference to note burnBlocked is deprecated at Cobalt but still dialable, and to link to the new page.
• Added the new page to the Cobalt group in docs.json.

Context

Mirrors the base-std changelog entry added in base/base-std#199.

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