Tidy README

[dmytro-khmara]

Summary

• Same readability pass applied across the StrEnum family (StrEnum, StrEnum.EntityFrameworkCore, StrEnum.AspNetCore). README copy only, no code changes.
• Adds sub-headings inside Usage so the section isn't a wall of prose:
  • ### Defining a string enum and a model
  • ### Configuring the serializer
  • ### Serializing to JSON
  • ### Deserializing from JSON
• Style tweaks: Allows for ... JSON serialization and deserialization → Lets you serialize and deserialize ... to JSON; The package targets .NET Standard 2.0 and can be used with System.Text.Json 4.6.0-10.* → tighter and en-dashed; You can install ... using → Install ... via; drops trailing-colon heading style (### Serialize to JSON: → ### Serializing to JSON).
• Code samples: rename the cryptic ctct variable to race; remove the duplicated JSON snippet in the Deserialize section (refers back to the JSON shown in the Serialize section instead).

Test plan

• Render the README to verify formatting and the new sub-headings
• Confirm code samples still match the package's actual API