Tidy README

[dmytro-khmara]

Summary

• Same readability pass as the StrEnum.EntityFrameworkCore tidy PR, applied to the Npgsql README for consistency.
• Allows to use → Lets you use; trailing-period fix; You can install ... using → Install ... via; trims conversational lead-ins (And call, Nothing stops you from, etc.); will produce / will look like → present tense.
• Adds #### Defining a string enum and an entity and #### Wiring it up sub-headings inside the text storage section so the first usage block isn't headless.
• Promotes the "UseStringEnums() is required in both modes" paragraph to a blockquote so it stands out.
• Drops the stale "Built on top of StrEnum.EntityFrameworkCore" lines (we removed that dependency before merge); replaces with a "see also" pointer to StrEnum.EntityFrameworkCore in the Acknowledgements section.
• No code changes.

Test plan

• Render the README to verify formatting and the new blockquote
• Confirm the dependency claim is consistent with the actual StrEnum.Npgsql.csproj (only StrEnum and Npgsql.EntityFrameworkCore.PostgreSQL)