[2.0] Clarify Generals.io bot task docs

[joyemang33]

[2.0] Clarify Generals.io bot task docs

Summary

Clarify the agent-facing documentation for the Generals.io bot arena task after the task landed on main.

This PR:

• Adds the upstream generals-bots gameplay GIF with MIT license credit.
• States the task goal explicitly: protect your own general and capture the opponent's general quickly and often.
• Documents the simulator rules visible to agents, including fog/visibility, movement, split moves, capture rules, army growth, truncation, and observation fields.
• Updates the root README 2.0 problem badge from 9 to 10 based on the current 2.0/problems directory count.

Please read CONTRIBUTING.md before submitting.

Type of Change

• New research problem
• New algorithmic problem
• New Frontier-CS 2.0 problem
• Bug fix
• Documentation update
• Other:

Testing

• Counted 2.0/problems/*: 10 problem directories.
• Ran git diff --check.

Checklist

• Code follows the project structure and conventions
• Self-review completed
• Documentation updated (if applicable)

CI Validation (for new problems)

Not applicable; this PR only updates documentation for an existing 2.0 task and the root problem-count badge.