Tidy README

[dmytro-khmara]

Summary

• Same readability pass applied across the StrEnum family (StrEnum, StrEnum.EntityFrameworkCore). No code changes, README copy only.
• Adds three sub-headings inside Usage so the section isn't a single wall of prose:
  • ### Registering the binder
  • ### Defining a string enum and a model
  • ### Binding string enums in controllers
• Style tweaks: Allows to use → Lets you use; Supports ASP.NET Core 6-10 → Supports ASP.NET Core 6 – 10 (consistent en-dash spacing); the v2.0.0 footnote is folded into the same line; You can install ... using → Install ... via; fixes the double-space typo in If you're using WebApplicationBuilder, add the call; tightens the awkward "add the call to ... into your Program.cs" → "call ... in Program.cs".
• Code samples: fixes [FromQuery]Sport (missing space), {...} → { ... }, and the 5-space indent in the Startup.cs snippet → 4-space.

Test plan

• Render the README to verify formatting and the new sub-headings
• Confirm code samples still compile-look correct (no logic changes)