WIP - docs: reorganize ColdFront docs into the standard pgedge nav structure

[susan-pgedge]

Summary

• Reorganizes mkdocs.yml's nav into the standard pgedge-docs template, preserving every original page as a flat link list on the new Source page.
• Splits installation, configuration, and Lakekeeper/object-store setup into dedicated guides, and splits day-to-day usage by operating mode (Tiered, Decoupled, Standalone Partitioned), with a new Administration section for shared operational topics (caveats, vended credentials, reading and writing, supported types, distributed setup, compaction).
• Converts the Using ColdFront and Architecture Guides sections to real index.md pages so their nav headers open the intro content directly, with no duplicate nav entry.

Test plan

• mkdocs build --strict passes with no warnings
• Spot-check the nav in a local preview (mkdocs serve): Using ColdFront, Architecture Guides, and Administration all render and link correctly
• Confirm README.md and docs/index.md still list every guide

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[codacy-production]

Up to standards ✅

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TIP This summary will be updated as you push new changes.}