Update BarChart and LineChart demos to enhance documentation and add stacked line chart example

[gvreddy04]

This pull request improves the Bar and Line Chart demo documentation and code samples to provide clearer, more consistent guidance on advanced features like stacking, data labels, and plugin usage. It also introduces a new stacked line chart example and streamlines the initialization of chart data and options in the demo components.

Documentation improvements:

• Updated the Bar Chart and Line Chart documentation to clarify how to enable and customize data labels, now recommending passing the ChartDataLabels plugin via the InitializeAsync method and providing more precise customization instructions.
• Added a new "Stacked line chart" section to the Line Chart documentation, explaining how to use the Stack property and stacked Y-axis for layered line/area charts, with step-by-step usage instructions.
• Improved section titles for consistency (e.g., "Tick configuration" instead of "Tick Configuration", and "Dataset level delay" instead of "DataSet level delay").

Demo code enhancements:

• Refactored the Bar Chart and Stacked Bar Chart with Data Labels demo components to initialize ChartData and BarChartOptions in a more concise and idiomatic way, reducing boilerplate and improving readability.
• Ensured that the correct plugin enabling and option customization for data labels are demonstrated, including setting label color and passing the plugin name during initialization.

New example:

• Added a new LineChart_Demo_07_Stack.razor demo that showcases a stacked line/area chart using the Stack property and stacked Y-axis, with multiple datasets representing device types.