docs(contest_2026): clarify 50-yuan deposit and merge flow sections

[tanghao-xiaomi]

Changes

zh-cn/contest_2026/contest_overview.md, two edits:

1. Benefits — free hardware: deposit clarified from "small deposit" to 50 yuan, and noted that it is paid via Feishu transfer to the dev-board operator in the contest group. Now consistent with the group announcement.
2. Merged "参赛流程" and "参赛全流程（从报名到获奖）": the two sections were adjacent, similarly named, and duplicated the schedule, which was confusing. Combined into a single "## 参赛流程" section — schedule table first, then the 10-step action checklist (content unchanged), removing the duplicate heading.

Verification

• Single file changed; section structure intact (no duplicate headings after merge).
• The 10-step flow and all doc links are preserved.