Add consolidated Feature Overview to README

[themuffinator]

Motivation

• Provide a single, concise area that outlines the project's major gameplay, multiplayer, and administrative features.
• Make it easier for contributors and server operators to quickly understand mapping, items, monsters, match systems, and UI improvements.
• Surface important modernization notes such as the move to modern C++ and server-side client configs.
• Improve discoverability of features like match export, auto-ghost recovery, and advanced map management for documentation readers.

Description

• Inserted a Feature Overview subsection under Core Highlights in README.md containing a bulleted list summarizing major features and systems.
• The list highlights mapping and entity expansions, new items and monsters, match state and management, map queuing, gametypes and modifiers, HUD/announcer, stats export, admin controls, and the modern C++ rewrite.
• This is a documentation-only change limited to README.md with no code modifications.
• The change was committed with the message Update README feature overview.

Testing

• No automated tests were executed because this is a documentation-only change.

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Codex Task