SEO: Optimize README and POM descriptions for java.util.json discoverability

[simbo1905]

What

• Update README title and opening paragraph to target searches for "java.util.json" + "Java 21"; add links to the OpenJDK sandbox "json" branch and design paper.
• Enrich module POM metadata by adding/expanding and with clear, keyword-rich summaries:
  • Core: backport of OpenJDK sandbox java.util.json API for Java 21+, immutable JSON value types, parsing utilities.
  • Schema validator: experimental JSON Schema 2020-12 validator built atop the backport; runs the official test suite.
  • Compatibility suite: executes JSON Test Suite to track standards conformance for the backport.
  • API tracker: compares upstream OpenJDK sandbox API to the backport and reports differences.
• Keep artifact coordinates as-is; do not rename modules.
• No behavioral changes and no build changes beyond metadata/doc edits.