JsonPath docs cleanup and mvnw commands #126

We want module docs to be human-facing and low-noise:

• Promote json-java21-jsonpath module documentation to README.md (not ARCHITECTURE.md).
• Keep the README focused on public usage (avoid test layout / internal implementation details).
• Use ./mvnw in human READMEs (agent mvnd/mvn wrapper stays in AGENTS.md).
• Ensure JsonPath docs/examples emphasize compiling the path and querying parsed JsonValue (avoid repeated parsing).