docs(input-mapping): AJDA-2714 document description and schema fields in input table manifest

[odinuv]

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Linear issue: https://linear.app/keboola/issue/AJDA-2714/input-mapping-read-description-primarily-from-table-definition

Jira issue(s): PROOF-XXX

Documents the two new fields the input mapping now writes into /data/in/tables manifests (per keboola/platform-libraries#533):

Changes:

• Document the new top-level description field — read primarily from the table's native description, falling back to KBC.description metadata, omitted when empty.
• Document the new schema array — per-column name, data_type (base plus the table's backend type with optional length/default, omitted for non-typed columns), nullable, primary_key, and description (native field with KBC.description fallback). Links to the output-manifest native-types page, which shares the same schema structure.
• Fix the example's table-metadata key from table_metadata to metadata (matching the code and the surrounding prose) and remove a stray trailing comma.
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[linear]

AJDA-2714