Add reusable Timeline component

[sydbautista]

What changed

• add a reusable Bootstrap-based Timeline component with vertical and horizontal orientations
• support left, right, and alternating vertical layouts with responsive mobile behavior
• add timeline item metadata, Bootstrap/Font Awesome/custom icons, images, badges, custom render fragments, and accessible action buttons
• add demo pages for layouts, rich content, callbacks, and dynamic collections
• add API documentation routes and navigation entries

Why

Applications frequently need to present chronological activity or milestones. This provides a consistent, reusable component that works in Blazor Server and WebAssembly without JavaScript.

Validation

• dotnet build blazorbootstrap/BlazorBootstrap.csproj --no-restore (net6.0, net8.0, net9.0, and net10.0)
• dotnet build BlazorBootstrap.Demo.RCL/BlazorBootstrap.Demo.RCL.csproj --no-restore
• both builds completed with zero errors; existing repository warnings remain

Review note

The documentation screenshot constant temporarily uses home.png until a dedicated Timeline screenshot is added.