[R] Developer Docs followups

[asfimport]

To dos to add later:

• Check if withr::with_makevars(list(LDFLAGS = ""), remotes::install_github(...) is sufficient instead of withr::with_makevars(list(CPPFLAGS = "", LDFLAGS = ""), remotes::install_github(...)
• add a latest nightly downloader helper function https://github.com/apache/arrow/pull/9898#discussion_r612891598
• Add description of docker + crossbow https://github.com/apache/arrow/pull/9898/files#r613590007

Reporter: Jonathan Keane / @jonkeane

_{Note: This issue was originally created as ARROW-12372. Please see the migration documentation for further details.}

[asfimport]

Todd Farmer / @toddfarmer: This issue was last updated over 90 days ago, which may be an indication it is no longer being actively worked. To better reflect the current state, the issue is being unassigned. Please feel free to re-take assignment of the issue if it is being actively worked, or if you plan to start that work soon.