Add DHTMLX Gantt and Diagram

[plazarev]

Checklist:

• [x] I've read and understood Contributing Guidelines.
• [x] I've added the new resource at the end of its section.
• [x] This resource is out there for a while, and actively maintained.
• [x] This resource is popular enough and has at least a few hundred stars on GitHub.

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1. DHTMLX Gantt is a JavaScript component for creating interactive Gantt charts to manage projects, tasks, and deadlines. The open-source version supports features like drag-and-drop scheduling, dynamic loading, and exporting to PDF/Excel.

2. DHTMLX Diagram allows building different types of diagrams and flowcharts, such as org charts, decision trees, and BPMN diagrams. It provides a drag-and-drop editor, customizable shapes, and integration with modern frameworks. ...