docs(contest): add FAQ on requesting MiMo Token Plan top-up

[tanghao-xiaomi]

What changed

Added a new FAQ entry to 《大赛总览》 explaining how to request a MiMo Token Plan top-up when running low:

1. First submit AI coding logs (via ai_coding_log_guide.md) to logs/ in the team's contest repo, to confirm the token usage is genuinely tied to contest development.
2. Provide a new MiMo account not currently within an active Token Plan period (new registration or an unused teammate account), since top-ups can only be injected into accounts without an active plan — same constraint already documented in the adjacent "已有 MiMo 订阅套餐" FAQ.

Placed directly after the existing "已有 MiMo 订阅套餐，还能申领大赛的 Token Plan 吗？" entry, since both share the "no active plan" account constraint.

Testing

Documentation-only change; no build required.