docs: Document the ORM upsert methods

[marcelomendoncasoares]

Summary

upsertRow and the batch upsert are generated on every table model but were missing from the CRUD page, so there was no documented way to insert-or-update a row.

Changes

• Add an "Upsert" section to the CRUD page, right after Create and its conflict handling.
• Extend the shared Company example model with a unique name, address, employeeCount, and isActive, so the conflict examples have something to conflict on. The address field was already used by an existing example on the page but missing from the model.
• Show upsertRow with conflictColumns, and explain the default update set.
• State that the primary id column can be a conflict target.
• Document updateColumns and updateWhere, and explain why upsertRow returns a nullable value.
• Document the batch upsert: atomicity, transaction support, a result that can be shorter than the input, and noReturn.
• Explain how non-persistent fields behave.

Reference

• feat: Upsert serverpod#4832
• UPSERT support? serverpod#232
• fix: Allow using the id as conflict target on upsert and upsertRow serverpod#5342, fix: Encode column values correctly in upsert insert queries for SQLite serverpod#5353, fix: Fix upsert with updateWhere that matches rows on SQLite serverpod#5428