docs: add AGENTS.md and improve documentation discoverability

[Sunny6889]

What does this PR do?

Documentation-only change (no code or build changes) that makes the project easier to navigate for new contributors and AI coding assistants:

• Adds AGENTS.md at the repository root — a concise contributor/AI guide covering build & test commands, the module layout and one-way dependency direction, the hard constraints (cross-JVM determinism, DB Session usage, actuator / protobuf / API / thread rules), coding & security rules, and the commit convention.
• Adds a Documentation section to README.md linking the guides under docs/ (configuration, modular architecture & deployment, implementing a customized actuator) and the authoritative Protobuf protocol document.
• Moves the root protocol document Tron protobuf protocol document.md → docs/protobuf-protocol-document.md (pure rename, no content change): removes the spaces from the filename and consolidates it with the other guides under docs/.
• Marks the two outdated protocol copies under protocol/src/main/protos/ (English / Chinese, last updated 2022) as superseded, pointing readers to the maintained document.
• Adds a short security note to docs/configuration.md on protecting a Super Representative's block-producing key (prefer the encrypted keystore, restrict file permissions, never commit a real key).

Why are these changes required?

• No single entry point for building/navigating the codebase, and the docs/ guides were not linked from the README — hard for contributors and AI assistants to discover.
• The root protocol document's filename contained spaces (breaks relative Markdown links in several editors/renderers and is awkward for tooling), and the stale copies under protocol/src/main/protos/ did not point to the maintained version.
• The configuration reference gave no guidance on protecting the Super Representative block-producing key.

This PR has been tested by:

• Unit Tests: N/A — documentation only, no code or build changes.
• Manual Testing:
  • Verified every relative link resolves to an existing file and the Markdown renders correctly.
  • Confirmed git records the protocol-document change as a pure rename (R100) with no content diff.
  • Verified the rename does not break the developer documentation: searched all references in tronprotocol/documentation-en (the old filename in any form, java-tron blob/tree/raw links, and .md links) — the file is not referenced there, so no links break.

Follow up

• Consider linking AGENTS.md from the developer documentation site.

Extra details

• The maintained protocol document moved: Tron protobuf protocol document.md (repo root) → docs/protobuf-protocol-document.md. Rename only; content unchanged.
• 6 files changed, +118 lines; no source code, build scripts, or config defaults touched.