docs: EXPOSED-578 Extend the Entity definition topic and add examples

[vnikolova]

Description

Extended the Entity definition topic in the following ways:

• Added new sections with more detailed documentation for Entity type, EntityClass, Properties, Immutable entities and Class method overrides.
• Added code examples to the exposed-dao project for method overrides and immutable entities.
• Moved the content from the Field transformation topic to Entity definition as a separate sub-section.

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Type of Change

Please mark the relevant options with an "X":

• Bug fix
• New feature
• Documentation update

Related Issues

EXPOSED-578

[bog-walk]

Code is missing a few pieces to compile:

class EntityWithUInt(id: EntityID<Int>) : IntEntity(id) {
    var uint: UInt by TableWithUnsignedInteger.uint.transform({ it.toInt() }, { it.toUInt() })

    companion object : IntEntityClass<EntityWithUInt>(TableWithUnsignedInteger)
}

[vnikolova]

Fixed and moved to a separate entity file. This function seems to be deprecated though. Is there a better example we can show here?

[bog-walk]

Yes, the old DAO transform() is currently deprecated but it will not be removed from the API entirely until some issues with caching are fully resolved.
If all goes well, this entire 'Fields transformation' section would become redundant and we could redirect users to the DSL transform section.
But if the issues prove problematic, then it would be good to keep this example up in the event we rollback deprecation (or implement some other DAO-level change).