sysvar: introduce tidb_enable_ddl as an introduce scope sysvar

[CbcWestwolf]

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What is changed, added or deleted? (Required)

Add description about tidb_enable_ddl.

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• v5.4 (TiDB 5.4 versions)
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What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s): config, sysvar: add config instance.enable_ddl and sysvar tidb_enable_ddl tidb#35425

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• Delete files
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• Might cause conflicts after applied to another branch

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[CbcWestwolf]

Move to #10978