docs(intent): renamedFrom / dropped - evolving a table that already holds data

[delchev]

Documents the two intent keys for evolving the table of an entity whose application already holds data (expand/contract), shipped in eclipse-dirigible/dirigible#7674 (issue eclipse-dirigible/dirigible#7635):

• field-level renamedFrom: <old field name> - the publish renames the live column in place, so the values move with the new name; it acts only while the old column exists and the new one does not, so it is harmless to leave in the file. On SQL Server (no in-place rename) the new column is added nullable, the values are copied, and the old column is kept.
• entity-level dropped: [names] - former field / to-one relation names whose columns the publish removes, data included.
• the default: a field simply deleted from the file keeps its column and values - logged at WARN (table, column, owning artefact), counted as orphanColumns on the artefacts health component at /actuator/health, a kept NOT NULL column relaxed to nullable on H2/PostgreSQL/Snowflake and left NOT NULL with a WARN on MySQL/MariaDB/SQL Server/HANA; DIRIGIBLE_DATABASE_DROP_UNDECLARED_COLUMNS=true as the dev-instance opt-in (a ::: tip).
• the Generate-time refusals (a ::: warning).

Changes: a new section renamedFrom / dropped - evolving a table that already holds data in docs/help/intent/dsl-reference.md (before defaultValue), its index row, and a renamedFrom row in the field table of docs/help/intent/intent-file.md.

npm run docs:build is green; the built HTML carries id="renamedfrom-dropped-evolving-a-table-that-already-holds-data" and the index and intent-file links resolve to it. No em/en dashes in the diff.

The neutral specification counterparts: IntentFile/intent-specification#96 and IntentFile/intentfile.github.io#65.

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