[R][Docs] Document writing bindings between C++ code and R functions

[asfimport]

This is closely related to, but does not duplicate ARROW-14757 as this is at a different level of detail

Reporter: Nicola Crane / @thisisnic

Related issues:

• [Doc] Steps in making your first PR - R bindings (relates to)

_{Note: This issue was originally created as ARROW-14794. Please see the migration documentation for further details.}

[asfimport]

Nicola Crane / @thisisnic:
Actually, never mind, it would totally duplicate that other ticket