[4/5] VSA: Slim Configuration to YAML schema; organize root namespaces

[Mpdreamz]

Why

Elastic.Documentation.Configuration was accumulating types that have nothing to do with YAML schema parsing — a domain interface, a security utility, CLI options, and dead code. Elastic.Documentation's root namespace had the same drift: version types, changelog enums, and a CLI options class all living next to cross-cutting primitives with no structural signal about their purpose. Reviewers and future contributors had no way to distinguish schema types from infrastructure types without reading each file.

What

Elastic.Documentation.Configuration is trimmed to YAML schema and deserializer wiring only. IEnvironmentVariables and SymlinkValidator move to Elastic.Documentation (domain primitives); CrossLinkEntry and DocSetRegistry move into the existing Builder/ sub-namespace; the dead LenientStringListConverter (zero references, contradicted by its own doc-comment) is deleted. ElasticsearchEndpointConfigurator stays in Configuration due to a dependency cycle (Configuration → Tooling already exists).

Elastic.Documentation's root namespace is pruned to genuine cross-cutting primitives. ChangelogEntryType and ChangelogEntrySubtype move into the existing ReleaseNotes/ sub-namespace; SemVersion, VersionSpec, and VersionOrDate move to a new Versions/ sub-namespace; GlobalCliOptions moves to Elastic.Documentation.Tooling (preserving the Elastic.Documentation namespace via pragma, so ~60 consumer files need only a using update, not a project-reference change).

All consumer changes are mechanical using directive additions — no logic was touched.

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Part of a move to vertical slice architecture to be able to cleanly lift search contract:

1. [1/5] VSA: Merge Elastic.Documentation.Api.* into single project #3333 — Merge Elastic.Documentation.Api.* into single project
2. [2/5] VSA: Collapse Gateway+Usecase into *Service per feature #3335 — Collapse Gateway+Usecase into *Service per feature
3. [3/5] VSA: Slim Elastic.Documentation, extract heavy deps to Tooling #3345 — Slim Elastic.Documentation, extract heavy deps to Tooling
4. [4/5] VSA: Slim Configuration to YAML schema; organize root namespaces #3347 — Slim Configuration to YAML schema; organize root namespaces
5. [5/5] VSA: Extract Elastic.Documentation.Search.Contract as zero-dep library #3350 — Extract Elastic.Documentation.Search.Contract as zero-dep library