docs: clarify architecture-specific JDK requirements

[halibobo1205]

What does this PR do?

• Clarifies the current JDK restrictions enforced when both building and running java-tron:
  • x86_64 / amd64 requires JDK 8.
  • ARM64 / aarch64 requires JDK 17.
• Replaces the architecture-ambiguous JVM options example with a heap-size-only example.
• Explains that generated distributions already include GC options appropriate for the build architecture and required JDK.

Why are these changes required?

The JDK mapping is not merely a build-toolchain recommendation. It is enforced at build time in build.gradle and again when a FullNode starts through Arch.throwIfUnsupportedJavaVersion().

The existing documentation could be interpreted as allowing either JDK 8 or JDK 17 regardless of CPU architecture. This led to a reasonable but incorrect assumption that a node could run with JDK 17 on x86_64.

This PR makes the currently supported build and runtime combinations explicit and helps prevents users from copying incompatible GC options between JDK 8 and JDK 17 deployments.

This PR has been tested by:

• Unit Tests: Not run (documentation-only changes).
• Manual Testing:
  • Verified the documented requirements against the build-time and FullNode startup checks.
  • Verified the architecture-specific generated JVM option files.
  • Ran git -c core.whitespace=cr-at-eol diff --check.

Follow up

• Coordinate an equivalent clarification for the related pages on developers.tron.network.
• JDK 17 support on x86_64 remains planned for a future release.

Extra details

ARM64 / aarch64 support is available starting with GreatVoyage-v4.8.1.

Refs #6895

[waynercheung]

LGTM