feat: included sphinx mermaid python requirement

[Shubhangi-Nikam26]

Summary
Added Mermaid support to the SCORE Sphinx documentation toolchain so .. mermaid:: blocks are recognized and rendered during documentation builds.

Problem
The documentation toolchain did not include Mermaid support in its Sphinx setup. As a result, builds failed when encountering mermaid directives because they were treated as unknown directives.

Changes

1. Add sphinxcontrib-mermaid to the documentation Python requirements.
2. Expose the dependency in the Bazel documentation tooling target.
3. Enable sphinxcontrib.mermaid in the generated Sphinx configuration.