Document complex types

[AndriySvyryd]

See

• https://learn.microsoft.com/en-us/ef/core/what-is-new/ef-core-8.0/whatsnew#value-objects-using-complex-types
• Add complex types to Metadata efcore#31003
• Only discover complex types when configured through pre-convention model configuration or annotation efcore#31399
• Fix saving null complex properties with nested required complex properties and other cleanup efcore#36472
• Add support for complex type discriminator. efcore#35744
• Add support for optional complex types to model building efcore#35614
• Implement optional complex types efcore#36502
• Add model building support for complex type collections efcore#35905
• Add change tracking support for complex collections efcore#35962
• Add complex collection support to PropertyValues efcore#36366
• Allow configuring the column type with ToJson() efcore#36478
• Complex JSON query support efcore#36355

Compare with owned types (#4265):

Owned types:

• Have identity
• Can only be mapped to a refence type
• Are mutable, can have navigations
• Can be mapped to different tables
• Can be mapped as optional with table sharing

Complex types:

• No identity, single instance can be referenced from different entities
• Can be mapped to a reference or value type
• Recommended to be immutable (especially for value types)
• Need to be mapped to the same table (with the exception of entity splitting). Collections need to be mapped to JSON in relational databases
• Can be mapped as optional with table sharing, but only if it has at least one required property