feat: included sphinx mermaid python requirement

[Shubhangi-Nikam26]

Summary
This PR makes Mermaid support configurable in rules_score and keeps it disabled by default, as requested for S-Core.
When Mermaid is not enabled, Sphinx does not load the Mermaid extension.
When Mermaid is enabled, the extension is loaded and available for docs that use Mermaid directives.

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
Added a new Bazel build setting: enable_mermaid (default False)
Updated Sphinx conf template generation to pass ENABLE_MERMAID from Bazel
Updated conf templates to append sphinxcontrib.mermaid only if ENABLE_MERMAID is true
Add sphinxcontrib-mermaid to the documentation Python requirements

[hoe-jo]

If it works only with sphinx options, I guess we don´t need to bake it into Rules Score, maybe you define you own Sphinx Toolchain - as we did in communication repo: https://github.com/eclipse-score/communication/tree/main/docs/sphinx - like this you can completely customize your build?