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ARROW-18391: [R] Fix the version selector dropdown in the dev docs (#14800)

For context, this overrides the default navbar HTML with a custom version which makes a single change - adding a span called "version" which the JS will override.  An identically-named span existed in the template used with Bootstrap 3, which is why no JavaScript changes were needed.  This approach is almost identical to that in this PR to the pkgdown package which implements the same thing: https://github.com/r-lib/pkgdown/pull/2072 

I built this locally, and it appears to have successfully added the dropdown back in.

![image](https://user-images.githubusercontent.com/13715823/205083695-e718c7e5-1dc1-4742-9ce8-f773611b8ab7.png)


Authored-by: Nic Crane <thisisnic@gmail.com>
Signed-off-by: Nic Crane <thisisnic@gmail.com>