Document Redis data layout

[Mirochill]

What is the purpose of the change

This pull request documents the Redis key and value layout used by RedisSink and expected by RedisSource.

Closes #250

Brief change log

• Replace the Redis connector TODO with a data layout section.
• Describe the default key structure, Redis value types, update behavior, null handling, and timestamp-field handling.

Does this pull request potentially affect one of the following parts:

• Dependencies: no
• The public API: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? docs

Validation

• git diff --check
• git diff --cached --check
• git diff --check HEAD~1..HEAD
• git show --check --stat --oneline HEAD
• Not run locally

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