resources/cdnfresh.txt: missing 7.5 manual

[shawn0915]

Change Request

This repository is ONLY used to solve problems related to DOCS-CN.
For other issues (TiDB, TiKV, etc), please move to other repositories.

Please answer the following questions before submitting your issue. Thanks!

1. Describe what you find is inappropriate or missing in the existing docs.

https://github.com/pingcap/docs-cn/blob/master/resources/cdnfresh.txt#L5

cdn 中缺失 7.5 的手册
tidb-v7.5-zh-manual.pdf
tidb-v7.5-en-manual.pdf

2. Describe your suggestion or addition.

建议增加相关pdf

3. Provide some reference materials (documents, websites, etc) if you could.