[R] Tidy up the pkgdown articles site index

[asfimport]

We could better organise the different articles we have to make it easier for users to find the right info

Reporter: Nicola Crane / @thisisnic

_{Note: This issue was originally created as ARROW-17528. Please see the migration documentation for further details.}

[asfimport]

Stephanie Hazlitt / @stephhazlitt:
We could consider a few broad categories for the first level e.g. Developers (already there), Installation, Users.

[github-actions]

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