update DM document

[IANTHEREAL]

• deploy, remove server_id and use source_id in sample config file
• sharding overview and design detail
  • Updated and added in tools, media, readme: add DM shard merge #887
• troubleshot shard ddl lock
  • Added in tools: update descriptions of handling sharding ddl locks manually #876
• put Precheck the upstream MySQL instance configuration of task management into a separate document
  • Add the precheck to the deployment doc (after confirming with @GregoryIan)
• add relay log introduction
• update troubleshot
• add caution for restarting dm
  • Already added here.
• add sample task scenario

[IANTHEREAL]

@lilin90 I simply sorted out the documents that need to be modified, and you can adjust them. I will help you to modify them next week, and I hope that you can take the time to deploy and use DM to confirm this document is useful. 😃 Let's cheer together.

[lilin90]

Thanks! Currently, we already have the English version relay log and simple task scenario documents, and I added them to TOC in #869. And I saw your PR #873. We can discuss the TOC later.

[IANTHEREAL]

@lilin90 you can add the items that you found while you try DM into this issue, easy to trace them

[lilin90]

@GregoryIan Got it. I'll add them later.