Documenting binary encoding used with Canal-JSON (#14774)

[ti-chi-bot]

This is an automated cherry-pick of #14774

First-time contributors' checklist

• I've signed Contributor License Agreement that's required for repo owners to accept my contribution.

What is changed, added or deleted? (Required)

I added documentation that describes how the binary data found in BINARY, VARBINARY, and BLOB columns is encoded when using the Canal-JSON format.

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• v7.2 (TiDB 7.2 versions)
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What is the related PR or file link(s)?

None

• This PR is translated from: Documenting binary encoding used with Canal-JSON docs#13832 N/A
• Other reference link(s): N/A

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

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