Add docs for SUM_INT, MAX_COUNT, and MIN_COUNT

[windtalker]

What is changed, added or deleted? (Required)

• Add docs for the TiDB-specific aggregate function SUM_INT().
• Add docs for MAX_COUNT() and MIN_COUNT(), including syntax, return value, NULL handling, DISTINCT limitation, window-function usage, and TiFlash pushdown support.
• Update the window function and TiFlash pushdown docs accordingly.

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)

What is the related PR or file link(s)?

• This PR is translated from: Add docs for SUM_INT, MAX_COUNT, and MIN_COUNT docs#23199
• Other reference link(s):
  • *: add aggregate function sum_int (#66085) tidb#69457
  • *: add aggregate functions min_count and max_count tidb#69642

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[qiancai]

@windtalker, would you please involve a tech review for this PR? Btw, the code change of this PR is only made in the master branch of the pingcap/tidb repo, right? If so, I will not cherry-pick this PR to other doc branches.

[ti-chi-bot]

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