[Docs] Add canonical url to the pkgdown (R) docs

[asfimport]

A canonical URL is the URL of the page that Google thinks is most representative from a set of duplicate pages on your site. More info can be found here: https://www.authoritylabs.com/solving-canonical-problems/

Reporter: Nicola Crane / @thisisnic
Assignee: Nicola Crane / @thisisnic

_{Note: This issue was originally created as ARROW-14339. Please see the migration documentation for further details.}

[asfimport]

Nicola Crane / @thisisnic:
I'm not sure this is important any more; whilst I copied this ticket as there was a similar one for the main docs, if I go to e.g. "https://www.google.com/search?q=site%3Ahttps%3A%2F%2Farrow.apache.org%2Fdocs%2Fr%2F+datasets", I only end up with links to the release docs and not individual version ones (perhaps the shorter URL without '/version_number/' or '/dev/' is sufficient hint to Google?)

@jorisvandenbossche Would you mind chiming in here about whether this is important for us to do for the R docs?

[asfimport]

Joris Van den Bossche / @jorisvandenbossche:
To be honest, I am no web expert, so no idea how important it is to set this canonical url (maybe it would only become important later, since right now the different versions are quite recent and almost no one is linking to those?)

If it is not straightforward to set this (in sphinx it was an available configuration option, so was easy to do), I think it is fine to table this for now.

[asfimport]

Nicola Crane / @thisisnic:
Thanks @jorisvandenbossche, I'll close this ticket for now and look to do this only if we see it's absence causing an issue.