docs: reorganize documentation and add sub-pages

[zlzGithub-0801]

What

Reorganize the Core Concepts and aenv CLI documentation into focused subpages.

Why

Several documentation pages had become too long and difficult to navigate. The CLI reference also needed to match the latest command implementation.

Related issue

N/A — no issue provided.

Scope and non-goals

Includes documentation structure, internal links, CLI parameter descriptions, and BuildKit template-build documentation. No runtime behavior, API, configuration, or storage format is changed.

Design and behavior changes

Documentation-only change. Core Concepts and CLI topics now use nested pages. User-facing template documentation focuses on commands, startup behavior, build capacity, and monitoring; worker lifecycle details are documented under Developer Internals.

Compatibility and operations

• Public API or generated protocol: N/A — unchanged.
• Configuration or defaults: N/A — existing settings are documented only.
• Snapshot manifest, artifact layout, or storage format: N/A — unchanged.
• Upgrade and rollback: N/A — documentation-only change.
• Host requirements, permissions, ports, or dependencies: N/A — unchanged.

Validation

• make fmt
• make clippy
• make test-unit
• Relevant Rust integration tests
• make -C services test (required when services/ changes)
• Generated clients/server regenerated with the documented make target
• Documentation updated
• Benchmarks or performance comparison completed

Skipped checks and reasons:

Code, generated protocol, service, integration, and benchmark checks were skipped because this PR changes documentation only.

Risks and reviewer notes

Low risk. Review the new navigation in docs/src/SUMMARY.md, the template build documentation, and the relocated BuildKit internals.

Checklist

• The PR contains one coherent change and no unrelated formatting or refactoring.
• New behavior is covered by tests, or I explained why testing is impractical.
• Logs and examples contain no credentials, tokens, or private registry information.
• I did not manually edit generated code without updating its source and regenerating it.

[github-actions]

✅ OpenCodeReview: Review complete: 0 finding(s) across 3 selected item(s).