GH-50726: [CI][Docs] Use bundled simdjson for documentation builds

[KHARSHAVARDHAN-eng]

Summary

The documentation CI jobs (debian-docs and conda-python-docs) fail during CMake configuration because simdjson_SOURCE is not explicitly set. As a result, CMake inherits the default dependency source (SYSTEM or CONDA) and attempts to use an installed simdjson package that may not satisfy Arrow's simdjson >= 4.0.0 requirement.

This change explicitly configures the documentation build services to use the bundled simdjson implementation by setting simdjson_SOURCE=BUNDLED in compose.yaml.

Testing

• Verified that docker compose config correctly propagates simdjson_SOURCE=BUNDLED for both debian-docs and conda-python-docs.
• Verified CMake dependency resolution with -Dsimdjson_SOURCE=BUNDLED, confirming that the bundled simdjson (v4.6.4) is selected instead of an older system package.
• No user-facing changes.

Checklist

• My change addresses the issue described in [CI][Docs] The docs job is failed by simdjson >= 4.0.0 #50726.
• The change is limited to the documentation CI configuration.
• No unrelated files were modified.

• GitHub Issue: [CI][Docs] The docs job is failed by simdjson >= 4.0.0 #50726

[raulcd]

There's already a PR for this issue.
@KHARSHAVARDHAN-eng can you fix the comments on your previous PRs before opening new ones, please? You have opened 9 PRs in a couple of days and keep opening new ones instead of fixing previous ones.
An you open new ones when there are other PRs from contributors already fixing.