Expand README with features section and API documentation

[bernardladenthin]

Summary

This PR significantly expands the README documentation to provide a more comprehensive overview of the library's capabilities and usage patterns. The changes reorganize the table of contents, add a dedicated Features section, and include new documentation for chat completion, embeddings/reranking, and raw JSON endpoints.

Key Changes

• Added Features section - Highlights key capabilities including text completion, chat completion, embeddings, reranking, infilling, tokenization, grammar conversion, raw JSON endpoints, model metadata access, and platform/acceleration support
• Reorganized table of contents - Restructured to include the new Features section as the first item, with updated numbering for all subsequent sections
• Expanded Documentation - Added three new subsections under Documentation:
  • Chat Completion with code examples showing streaming and blocking modes
  • Embeddings & Reranking with usage examples
  • Raw JSON Endpoints describing the available handler methods and server state management
• Removed outdated content - Removed the "Download" section with the JAR badge and the Gemma 3/4 support note
• Enhanced Infilling section - Kept existing content but positioned it after the new Chat Completion section

Notable Details

• Chat Completion examples demonstrate both streaming (generateChat()) and blocking (chatComplete()) patterns
• Embeddings section shows how to enable embedding mode and retrieve sentence embeddings
• Raw JSON Endpoints section documents the full set of handler methods and server management APIs
• All code examples follow the existing documentation style and patterns

https://claude.ai/code/session_01Phsbbq9JdFU24F9PGwG1wf