fix: add mermaid.js library for diagram rendering

[AliYmn]

This PR fixes the Mermaid diagram rendering issue in the documentation by explicitly adding the Mermaid JavaScript library. Even though Material for MkDocs has built-in Mermaid support in newer versions, explicitly adding the library resolves rendering problems with sequence diagrams and other Mermaid-based visualizations.

Changes:

• Added mermaid.js CDN link to mkdocs.yml
• Ensures proper rendering of all sequence diagrams in documentation

Before:

After:

[cunla]

What URL is the diagram not rendered? Looking here: https://django-tasks-scheduler.readthedocs.io/en/latest/ - I see the diagram fine.

[AliYmn]

@cunla It may be related to my browser location, because it does not load for me, and diagrams do not appear

I got 520 from: https://unpkg.com/mermaid@11/dist/mermaid.min.js
I got 200 from: https://cdn.jsdelivr.net/npm/mermaid@10/dist/mermaid.min.js

If you specify a CDN in your configuration, the system will use that CDN instead of unpkg., choosing a CDN will avoid these problems.

[cunla]

I have decided not to include this, since it seems to be an issue specific for your case.
And it should not download two versions of mermaidjs