[Docs] feat: Add documentation for the "Merging Designs" feature in Kanvas

[zihanKuang]

This issue is to create the documentation for the "Merging Designs" feature in Kanvas. The content needs to be revised to reflect recent changes to the UI and the merging process.

1. Reference the old file at https://github.com/meshery/meshery/commit/78d3a636030456e01b3f47252ba6d69c8cddf45c for the original content.
2. Go through the updated merging process in Kanvas. Note that the method and location for merging designs have changed.
3. Capture new screenshots and/or screen recordings of the updated process.
4. Write a clear description of the updated feature and its functionality.

Related Issue: https://github.com/meshery/meshery/issues/15609

[adan-shahid]

@zihanKuang I am interested in working on this issue. can you please assign it to me?

[saurabhraghuvanshii]

@adan-shahid I hope you raise Pr soon

[adan-shahid]

@saurabhraghuvanshii this given path is not showing anything. Can you provide me the previous documentation link?

[saurabhraghuvanshii]

@adan-shahid It woring on my side it is deletd file click on load diff to see content of deleted file https://github.com/meshery/meshery/commit/78d3a636030456e01b3f47252ba6d69c8cddf45c