docs: rewrite 'Encode metadata, not a handle to a live object' section

[Awesome06]

Which issue does this PR close?

Closes #1726

Rationale for this change

With recent PRs (such as #1723) modifying the examples to encode durable metadata, the "Encode metadata, not a handle to a live object" section in the extension guide needed a structural rewrite to reflect the current state of the codebase. It now correctly leads with positive examples and clarifies that the token registry used in the ForeignExecutionPlan arm is a marked workaround, not a pattern to emulate.

What changes are included in this PR?

• docs/source/extension-guide/codecs.md: Rewrites the section to point to datafusion-ffi-example (logical codec) and examples/distributed/storage-library as the primary, positive examples of encoding durable metadata.
• docs/source/extension-guide/codecs.md: Explains that a token registry is a consequence of an upstream defect ([Umbrella] FFI planner boundary: foreign-wrapped nodes, lost plan properties, and severed dynamic filters datafusion#25152) and isolates it to the ForeignExecutionPlan arm. Adds a bidirectional link to extension_codec_order.
• docs/source/extension-guide/checklist.md: Updates the checklist to clarify that the token workaround is now limited to "one arm of one codec" rather than a general practice in the repository's examples.

Are there any user-facing changes?

No (documentation improvements only).

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