Add explainer for Preserving dropEffect Values from dragover to drop Events

[roraja]

This PR adds an explainer documenting the fix for preserving dropEffect values set by web applications during dragover event handlers through to drop events, ensuring compliance with the HTML5 Drag and Drop specification.

What this explainer covers:

• The specification requirement that dropEffect should reflect the value set during the last dragover or dragenter event when the drop event fires
• The problem web developers faced when browsers overwrote the application-set dropEffect value before the drop event
• How this fix enables reliable drag-and-drop implementations where developers can determine the intended operation (copy, move, link) in their drop handlers
• Real-world use cases including file managers and custom drag-and-drop interfaces

[ashishkum-ms]

Can we add label also here so that tracking becomes easy?

[roraja]

Done