Document dehavior around default value handling, .NET type defaults and sentinel values

An extremely common issue is users getting confused about EF's behavior around default values: they e.g. set a default true value for a bool property, and are then surprised by not being to insert false.

Our documentation on [generated values](https://learn.microsoft.com/en-us/ef/core/modeling/generated-properties?tabs=data-annotations) doesn't explain how this all works, and although there are some bits and pieces in the change tracking section, we're missing a simple, basic section that users would find and understand. EF 8.0 also added the ability to configure the sentinel value, but apart from the [what's new section](https://learn.microsoft.com/en-us/ef/core/what-is-new/ef-core-8.0/whatsnew#sentinel-values-and-database-defaults) this has not been documented.