✨ feat(submission): Remove Round as body

[fan9704]

Aside from remove round also change response type from create serializer to whole submission object

https://github.com/fan9704/Drawing-Competition-Backend/issues/36

描述 Description

這支 PR 所做的事情，盡可能詳細取代本文字。 Replace this text and describe This PR doing(Better to be detail)

• 移除 Submission(提交程式碼) 需要帶入 round_id body
• 回傳格式更新成 Submission JSON 物件(開發請注意回傳型態)

相關的 Issues #(ticket number) / 無 Related Issues #(ticket number) / #36

變更的類型 Change Type

• [ ] Bug fix (non-breaking change which fixes an issue)
• [X] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

影響的 API Affect API

• [ ] Challenge
• [X] Submission
• [ ] Team
• [ ] Round
• [ ] 不影響 API No affect API
• [ ] 其他（請填寫） Other(Please enter it)

相關的 資源或參考文件連結 Related Resource/Document Link

If this PR related some resource/document please offer link

檢查清單 Check List

- [ ] 我已經在本地手動測試過，確保功能的完整性和穩定性 I have done fully correct manual test

• [X] 我對我的程式碼進行了註解，特別是在難以理解的地方 I have added comment in my code
• [X] 我已經更新了文件，或者我的更改不需要更新文件 I have commented in document
• [X] 我的 PR 有明確的標題與內容描述 My PR has right title and description