dm: the Start downstream TiDB commands in the DM quick start do not work #24490

The "Start downstream TiDB" step in dm/quick-start-create-task.md (lines 77-79) does not work as written:

wget https://download.pingcap.com/tidb-community-server-v8.5.0-linux-amd64.tar.gz
tar -xzvf tidb-latest-linux-amd64.tar.gz
mv tidb-latest-linux-amd64/bin/tidb-server ./

• The downloaded file and the extracted file have different names.
• tidb-community-server-v8.5.0-linux-amd64.tar.gz is a TiUP offline mirror package. It contains component tarballs, *.json metadata, and local_install.sh, but no bin/tidb-server, so changing only the tar line does not fix the step.
• tidb-latest-linux-amd64.tar.gz still exists, but it contains TiDB v5.0.1 (its top-level directory is tidb-v5.0.1-linux-amd64/), so going back to that file would start a very old TiDB, and the mv path would still be wrong.

The same text is in pingcap/docs-cn. The fix needs a decision on the intended setup, for example extracting the community server package and installing it with local_install.sh, or replacing these steps with tiup playground. I found this while reviewing the English docs for typos that affect the Japanese translation, and I left it out of the typo PR #24488 because it is not a simple typo.