[R] Fix matomo integration for R bookdown

This PR https://github.com/apache/arrow-cookbook/pull/283 added the matomo integration to the cookbooks.
After deploy we can see the matomo script on the different cookbooks and the index page but it does not seem to work with the R bookdown: https://arrow.apache.org/cookbook/r/index.html
This issue purpose is to track fixing that.

Review PR #283, which added the Matomo integration, then compare the deployed R bookdown index at arrow.apache.org/cookbook/r/index.html with the other cookbook pages. Trace how the R bookdown page is generated and define done as Matomo working there as it does on the other cookbooks.

Written by the indexing model from the issue text.