Polish object_store.md and regroup walkthrough nav

[susan-pgedge]

Summary

• Rewrote docs/object_store.md: removed step numbers from headings
  (per the updated pgedge-docs skill guidance against step numbers in
  headings), moved the "targets real cloud S3" / "no prior knowledge"
  paragraphs into Prerequisites, tightened the intro prose, clarified
  what docker compose up starts, and folded the three Lakekeeper
  setup steps (bootstrap, create warehouse, pre-create namespace) into
  flowing prose with First/Next/Then transitions instead of
  subheadings, for consistency with how the rest of the page handles
  sequential steps. Fixed two cross-references and the setup script's
  own stale step reference to match.
• docs/usage.md: fixed an ambiguous "Operating It" heading to
  "Operating the Partitioner", renamed "Prerequisites (Both Modes)"
  to "Configuring the Prerequisite Services", and updated a
  cross-reference anchor to match object_store.md's restructuring.
• mkdocs.yml: grouped the four walkthrough demo pages under a new
  "ColdFront Walkthroughs" nav subsection.

Test plan

• mkdocs build --strict passes locally
• Spot-check the rendered object_store.md and nav

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[codacy-production]

Up to standards ✅

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[susan-pgedge]

Closing without merging - pulled these changes into docs/standard-toc-experiment instead, as part of a broader nav restructuring. The object_store.md title conflict with the already-shipped PR #106 fix was resolved in favor of the shipped wording ("Configuring your Object Store").