Document Pattern clustering guidance

[CornetVincent]

What does this PR do? What is the motivation?

Motivation

Document the optional clustering guidance customers can provide when setting up Patterns, based on the source change in https://github.com/ddoghq/dd-source/pull/59556.

Changes

• Added an optional setup step for Clustering instructions in hugo/content/en/llm_observability/investigate/patterns.md.
• Explained that the guidance shapes how Datadog groups interactions and generates the resulting topic taxonomy.
• Clarified that leaving the field blank keeps grouping based on each interaction's main goal.

Testing

• Ran git diff --check.
• Tried to run vale hugo/content/en/llm_observability/investigate/patterns.md, but vale is not installed in this environment.

Merge readiness

• Ready for merge

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AI assistance

Used Bits Code powered by Codex CLI to draft and verify the documentation update.

Additional notes

None.

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Bits Code status: ✅ Done

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Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/dd/vincent.cornet/add-pattern-clustering-instructions/llm_observability/investigate/patterns