Restore Zensical documentation migration

[jonathan343]

Overview

This PR restores the Zensical documentation migration from #67, which was reverted in #73 due to excessive memory usage.

Zensical 0.0.53 includes an upstream improvement that shares cross-reference data between navigation clones, allowing the full documentation site to build successfully with reduced memory usage.

Summary

• Replace the MkDocs Material configuration with zensical.toml
• Update the documentation build and dependency lock workflow for Zensical
• Require and lock Zensical 0.0.53
• Generate Zensical navigation for all 18 clients
• Preserve modeled AWS SDK IDs such as API Gateway, QBusiness, and SageMaker Runtime HTTP2
• Restore the Zensical-specific client navigation behavior

Testing

$ make docs-install
$ make docs
$ make docs-serve

The complete documentation build finished with no issues:

• Zensical build time: 130.91 seconds
• Maximum resident memory: approximately 2.22 GB
• Swaps: 0

References

• Original migration: Migrate documentation to Zensical #67
• Revert: Revert "Migrate documentation to Zensical (#67)" #73
• Zensical memory improvement: Navigation clones copy the full Autorefs map for every page zensical/zensical#838
• Zensical 0.0.53: https://github.com/zensical/zensical/releases/tag/v0.0.53

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