docs: enhance README with quickstart snippets, sequence diagrams, and restructured navigation

[tomdesair]

Summary of Changes

This PR improves the developer experience and visual clarity of the README.md documentation:

• Dual-Protocol Badges & Intro: Added Maven Central and Java 17+ badges, and updated the intro to explicitly reference both Tus 1.0.0 and IETF Resumable Uploads for HTTP (RUFH).
• Key Features Summary & Table of Contents: Added high-level value proposition highlights and a clickable Table of Contents.
• Resumable Upload Sequence Diagrams: Added Mermaid flow diagrams for both Tus 1.0.0 and IETF RUFH showing request headers, chunk streaming, and handoff to application endpoints.
• Quickstart Code Snippets:
  • Added Gradle (Groovy & Kotlin) dependencies alongside Maven.
  • Added frontend Uppy/JavaScript configuration snippet.
  • Added Spring Boot @Bean initialization snippet in "1. Setup".
  • Added Spring MVC REST Controller snippet in "2. Processing an upload".
• Configuration Reference Table: Replaced the long with... bullet list with a structured markdown table.
• Upload Completion & Cloud Storage Guidance:
  • Added section "3. Handling Upload Completion & Retrieving Files" with a complete Spring controller example.
  • Added guidance on obtaining raw S3 object keys and Azure blob names directly for zero-download server-side processing.
• Restructured Sections: Extracted HTTP Digests (RFC 9530) and HTTP 104 Interim Responses into a dedicated "Advanced Usage" section.

[coveralls]

coverage: 94.572% (-0.03%) from 94.598% — feature/improve-readme into master