docs: comprehensive README update

[gnuhpc]

Summary

Comprehensive update of the root README.md to provide a complete and accurate overview of the project.

Changes

New Content

• Badges: Added crates.io, docs.rs, and license badges
• Features matrix: Full capability table for the core client (connection, admin, log tables, KV tables, data types, storage, WASM)
• Language bindings overview: Rust/Python/C++/Elixir comparison table with package, async runtime, and data format
• Project structure tree: ASCII directory layout with module responsibilities
• KV table example: New upsert + point lookup code example
• All 5 examples: Table with descriptions for log table, KV table, partitioned, prefix lookup variants
• Configuration reference: Key config options with defaults and descriptions
• Development section: Build/test instructions for all language bindings

Fixes

• Replaced outdated Args / ConnectionConfig API with current Config
• Examples now use proper Result<()> error handling instead of unwrap()
• Updated all code examples to use current builder patterns (Schema::builder(), TableDescriptor::builder())
• Clarified in the opening paragraph that this is a client SDK, not the Fluss server

Structure

• Clear section hierarchy with separators
• Quick Start with both log table and KV table examples
• Concise inline code blocks for easy copy-paste