Migration Guide: Custom validation

[hacksparrow]

Document the points of data access / validation and their differences in LB3 and LB4. Created out of #3935.

Acceptance Criteria

• Document the points where data (request / transaction) can be accessed and validated in LB3
• Document the points where data (request / transaction) can be accessed and validated in LB4

[agnes512]

Custom validation at different layers.

Better to have a lb4 documentation to talk about examples / use cases of custom validation at different layers first

[agnes512]

@dhmlau might have an article to talk about the use cases of validation.
Eidt: #4874

[agnes512]

The validation part of #4839 should be included in this story

[dhmlau]

See PR #4874