refactor(bazel): expose bundle-local Needs as JSON files

[AlexanderLanin]

Why

This is an internal Bazel improvement: it does not change the docs() or docs_bundle() API, or the generated documentation. Bundle-local Needs inventories are consumed as needs.json, but their Bazel target currently represents Sphinx's whole output directory. Publishing the JSON file directly gives downstream targets the exact artifact they consume and removes the dependency on Sphinx's private output layout. It also gives the provider-based follow-up a clear file artifact to carry.

What changed

• Keep the full Sphinx output directory private as an intermediate for local exports.
• Make each .__internal__.needs_local target expose its needs.json file directly.
• Resolve bundle-local inventories from that file in both external-Needs loading and duplicate-ID preflight.
• Update the target documentation and existing expected-output snapshots.
• Preserve the public :needs_json directory output for consumers of other Sphinx outputs such as metrics.json.

Closes #898

[github-actions]

Documentation preview for this pull request is available at:
pr-901: https://eclipse-score.github.io/docs-as-code/pr-901/