docs: standardize README to match other ParadeDB ORM repos

[philippemnoel]

Summary

Brings the efcore-paradedb README in line with the format used by django-paradedb, sqlalchemy-paradedb, and rails-paradedb so the ORM family presents consistently.

Header / branding

• Replace the local assets/paradedb-logo.png heading with the canonical ParadeDB readme SVG hosted in paradedb/paradedb.
• Add the standard tagline ("Simple, Elastic-quality search for Postgres") and top nav row (Website / Docs / Community / Blog / Changelog).
• Replace the previous ad-hoc badge row with the standard one: NuGet version, .NET version range, NuGet downloads, Codecov, License, Slack, Twitter.

Body

• Switch to the "## ParadeDB for Entity Framework Core" intro pattern with a single short paragraph linking to the docs getting-started anchor (#efcore).
• Add an EF Core row to the Requirements & Compatibility table.
• Drop the Configuration code snippet and Function Mappings table — neither of the sibling ORM READMEs has these. The Function Mappings table stays in NUGET-README.md (the package page), and configuration details belong on docs.paradedb.com.
• Keep the Examples list, Contributing, Support, Acknowledgments (Nandor Krizbai), and License sections.

Notes / follow-ups not in this PR

• The Codecov badge will display "unknown" until coverage upload is wired up (no .NET coverage step in ci.yml yet).
• The docs anchor #efcore on docs.paradedb.com/documentation/getting-started/environment may not exist yet; matches the convention used by the other ORMs (#django, #sqlalchemy, #rails). Update if the canonical anchor differs.
• assets/paradedb-logo.png is now unreferenced by the README but kept on disk — it's still packed into the NuGet package via src/Directory.Build.props (<PackageIcon>paradedb-logo.png</PackageIcon>).

Test plan

• Render preview on GitHub: header, badges, table, and lists display correctly
• lint-markdown workflow passes
• NuGet badge links resolve to https://www.nuget.org/packages/ParadeDB.EntityFrameworkCore