docs: fix TiKV RocksDB block-size defaults

[doc-claw-bot]

What is changed, added or deleted? (Required)

Fixes the outdated default RocksDB block-size value in the following TiKV Chinese docs:

• storage-engine/rocksdb-overview.md
• tune-tikv-memory-performance.md

These documents said the default block size is 64KB, but the TiKV config template uses 32KB for rocksdb.defaultcf and rocksdb.writecf.

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

Tips for choosing the affected version(s):

By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions (in Chinese).

• 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:
  • docs: fix TiKV RocksDB block-size defaults docs#23269
• Other reference link(s):
  • https://docs.pingcap.com/tidb/stable/tikv-configuration-file/#block-size
  • https://raw.githubusercontent.com/tikv/tikv/master/etc/config-template.toml
  • https://raw.githubusercontent.com/tikv/tikv/release-8.5/etc/config-template.toml

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]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: qiancai

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #21809.