refactor: centralize docs CLI path resolution

[AlexanderLanin]

Why

Sphinx builds currently resolve Bazel-provided paths in multiple layers. That makes bazel run, sandboxed bazel build, and direct invocation behave differently, and malformed input paths are reported later by Sphinx or an extension. A single configuration boundary makes the execution context explicit, validates required inputs early, and gives extensions one consistent source of configuration.

What changed

• Introduce typed DocsCliConfig path handling for direct, Bazel-run, and Bazel-build execution.
• Validate configured source directories and input files while loading the CLI configuration, while leaving declared Bazel outputs for Sphinx to create.
• Forward normalized paths through Sphinx configuration defines, including mounts, metamodel, known-good, and source-link inputs.
• Make Sphinx extensions consume their config values instead of migrated process environment variables.
• Preserve workspace runtime discovery for behavior that depends on the active Bazel context.
• Add regression coverage for build/run path resolution, early validation, mounts, and source-link consumers.

[github-actions]

Documentation preview for this pull request is available at:
pr-814: https://eclipse-score.github.io/docs-as-code/pr-814/