add duckplyr to dplyr backends #744
                
     Open
            
            
          
  Add this suggestion to a batch that can be applied as a single commit.
  This suggestion is invalid because no changes were made to the code.
  Suggestions cannot be applied while the pull request is closed.
  Suggestions cannot be applied while viewing a subset of changes.
  Only one suggestion per line can be applied in a batch.
  Add this suggestion to a batch that can be applied as a single commit.
  Applying suggestions on deleted lines is not supported.
  You must change the existing code in this line in order to create a valid suggestion.
  Outdated suggestions cannot be applied.
  This suggestion has been applied or marked resolved.
  Suggestions cannot be applied from pending reviews.
  Suggestions cannot be applied on multi-line comments.
  Suggestions cannot be applied while the pull request is queued to merge.
  Suggestion cannot be applied right now. Please check back later.
  
    
  
    
https://www.tidyverse.org/blog/2025/06/duckplyr-1-1-0/ proudly states duckplyr is now officially part of the tidyverse.
This PR adds 'duckplyr' under the 'dplyr backends' section of the index.md file, to accompany 'dbplyr' and 'dtplyr'.
I adapted the text from the duckplyr pkgdown site.