docs: Added documentation on DB row locking functionality

[FXschwartz]

Cover usage over PR - serverpod/serverpod#4680

@marcelomendoncasoares Docs submitted for the DB row locking functionality! Let me know what kind of changes you'd like made.

[Copilot]

Pull request overview

Adds new documentation for database row-level locking and expands the models documentation with support for JSON key aliasing, helping developers work more safely with concurrent DB updates and integrate with external JSON formats.

Changes:

• Added a new “Row locking” database concept page with examples for lock modes and lock behaviors.
• Documented the jsonKey model field option for JSON serialization/deserialization aliasing.
• Updated the models keyword reference table to include jsonKey.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
docs/06-concepts/06-database/13-row-locking.md	New guide explaining how to lock rows via reads and via lockRows, including lock modes/behaviors.
docs/06-concepts/02-models.md	New section documenting jsonKey and adds it to the keyword reference table.

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[marcelomendoncasoares]

Nice work with the docs, @FXschwartz! A couple of comments for improvement. Also, it seems that there is an unrelated change bundled in for the JSON key alias - that is already released.

[FXschwartz]

@marcelomendoncasoares Comments addressed and ready for another review!

[marcelomendoncasoares]

LGTM! 🚀

Thanks for this great contribution, @FXschwartz!