[DOCS-13261] Add drag and drop step documentation for browser tests

[git-thuerk-done]

What does this PR do? What is the motivation?

Fixes DOCS-13261

Adds documentation for the new drag and drop step in browser tests. Covers:

• Auto-recorded drag and drop steps (new subsection in Automatically recorded steps)
• Manually added drag and drop steps with full configuration details (new subsection in Manually added steps > Interaction)
• Step configuration options: drag/drop target elements, user-specified CSS/XPath locators, interaction point, and timing delays
• Known edge cases: ghost preview/drag overlay, nested DOM structures, drop target detected as dragged element
• Native vs. custom (mouse-based) drag and drop implementations
• Best practices

Note: Two TODO comments are left in the draft for follow-up:

1. Screenshots/video pending from Paris
2. GovCloud availability to be confirmed before publishing

Merge instructions

Merge readiness:

• Ready for merge

Additional notes

Placeholder {{< img >}} shortcodes are in place for a video (auto-recorded section) and screenshot (manual step section). These will be replaced once Paris provides the assets.

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/alicia/docs-13261-drag-and-drop-browser-tests/synthetics/browser_tests/test_steps