add docs for mpp partition table support

[shichun-0415]

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• This PR is translated from: add docs for mpp partition table support docs-cn#8698
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[shichun-0415]

@ran-huang 这个 PR 有一个术语：

动态裁剪/Dynamic Pruning
ZH: 在动态裁剪模式，TiDB 的每个算子都支持直接访问多个分区，省略 Union 操作，提高执行效率，还避免了 Union 并发管理的问题。
EN: In dynamic pruning mode, each operator supports direct access to multiple partitions. Therefore, TiDB no longer uses Union. Omitting the Union operation can improve the execution efficiency and avoid the problem of Union concurrent execution.

@windtalker 也请徐飞老师帮忙看看上面解释是否准确完整，谢谢。

[windtalker]

LGTM

[windtalker]

嗯，描述的没啥问题。

[shichun-0415]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: f715d86