Docs: add best practices section

[CommonGuy]

Add a few best practices to keep in mind while using Mapperly:

• Write very complex mappings by hand rather than trying to configure everything through Mapperly
• Use strict mappings (the default)
• Treat warnings as errors
• Do not write business logic in mapping code
• ...

[latonz]

Additional recommendations:

• When using Mapperly for requests and responses, it is generally best to maintain separate mappers: one for domain objects → API responses, and another for API requests → domain objects. This setup makes it straightforward to configure RequiredMappingStrategy: use Source for the request mapper and Target for the response mapper.
• Use a parameter name like source instead of a domain-specific name. This helps clearly identify the source object, particularly when the source and target types have similar names (e.g., Car vs CarDto or GetCarResponse).