Public docs_bundle is not self-contained because bundle examples include files outside docs/

## Problem

The public `//:docs_bundle` includes `docs/how-to/bundles/examples.rst` and the nested bundle example mounts. That page uses `literalinclude` paths such as:

```rst
../../../src/tests/docs_bzl/scenarios/nested_bundles/BUILD
../../../src/tests/docs_bzl/scenarios/data_files_runfiles/BUILD
../../../src/tests/docs_bzl/scenarios/external_bundle/BUILD
```

These files are outside the `docs/` source directory. The documentation builds successfully as the standalone `docs-as-code` project, but fails when its `docs_bundle` is consumed by another Sphinx project because `sphinx-mounts` enforces path confinement for mounted documents.

## Reproduction

Consume `@score_docs_as_code//:docs_bundle` from `eclipse-score/reference_integration` with `score_docs_as_code` 8.1.0:

```python
{
"bundle": "@score_docs_as_code//:docs_bundle",
"mount_at": "process_methods_tools/docs_as_code",
"attach_to": "process_methods_tools",
}
```

Run:

```text
bazel run //:docs
```

Observed result:

```text
sphinx-mounts: mounted doc process_methods_tools/docs_as_code/how-to/bundles/examples references a file outside its bundle root: .../score_docs_as_code+/src/tests/docs_bzl/scenarios/nested_bundles/BUILD is not under .../score_docs_as_code+/docs
```

A previous closed PR, #514, attempted to add support for `literalinclude` outside `docs/`; this is the same class of problem now exposed through the public external bundle and should be resolved either by packaging the included files into the bundle or by excluding these test-fixture examples from the exported bundle.

## Expected behavior

The public `docs_bundle` should be self-contained and mountable by another Sphinx project without path-confinement errors. The standalone documentation build and the external-bundle build should exercise the same contract.

Start with docs/how-to/bundles/examples.rst and the public //:docs_bundle definition, then reproduce the failure with the external-bundle configuration and bazel run //:docs. Trace which example files are exported outside docs/. Done means the standalone and external-bundle builds both succeed without path-confinement errors.

索引モデルが issue の本文から書いたものです。