[Exploration] docs: add Flox in Containers section

[djsauble]

Summary

• Creates a new top-level "Flox in Containers" section that unifies all container-related docs
• Adds Thin Containers docs (intro + getting started) for flox activate --sandbox and direct Docker/Podman usage
• Adds Containerize docs (intro + getting started) consolidating flox containerize content into a discoverable guide
• Migrates Imageless Kubernetes under the new section (no content changes, updated internal links)
• Adds a shared "How it works" tech page explaining Nix store volumes, environment resolution, and the flox/empty base image
• Frames flox containerize as the traditional/fallback approach, steering users toward thin containers and imageless K8s when possible
• Adds cross-references from existing concept and tutorial pages

Nav structure

Flox in Containers
├── Introduction
├── How it works
├── Thin Containers
│   ├── Introduction
│   └── Getting started
├── Containerize
│   ├── Introduction
│   └── Getting started
└── Imageless Kubernetes
    ├── Introduction
    ├── Installation (EKS, Self-managed, Troubleshooting, Upgrading, Uninstall)
    ├── Examples
    └── Configuration

Test plan

• Verify mkdocs builds cleanly with mkdocs serve
• Verify all nav items render correctly
• Verify no broken internal links (strict mode)
• Verify existing Imageless K8s pages render at new paths
• Review thin containers getting-started flow end-to-end
• Review containerize getting-started flow end-to-end

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