feat(bazel): resolve external Needs through providers

[AlexanderLanin]

Why

external_needs lets one documentation target use Needs from another. Today, our macro finds the inventory by deriving its private target name. If the input target is unsupported, the error can look like a missing generated target.

This draft makes docs targets identify their own local Needs file, so the build can reject unsupported targets early. It adds clearer validation, but little user-facing behavior; the private names are only used by our own rules. This is the implementation to review against the decision in #899.

What changes

• Public docs() and docs_bundle() targets publish a reference to their local Needs file.
• A build rule checks external_needs targets and writes a manifest of the inventories to load.
• The manifest and inventory files are passed through to Sphinx and the interactive docs command.

This PR is stacked on #901 and uses issue-898-needs-json-file as its base. Merge #901 first.