[R] Update developing.Rmd vignette

[asfimport]

Proposed changes:

• Make the subheadings in "developer environment" setup match the steps in the numbered bullets
• "at a specific ref" -> clarify
• include reference table of environment variables??
• add flowchart to " What happens when you R CMD INSTALL?" section?
• "so the installation process is easy" - rephrase from "is easy" to something that acknowledges it's only easy when it goes right!
• "it should look incredibly familiar" - rephase from "should" to "might" or similar
• Add something about the fact that we use tidyverse style and recommend styling code with styler

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

PRs and other links:

• GitHub Pull Request #10800
• GitHub Pull Request #10930

_{Note: This issue was originally created as ARROW-13403. Please see the migration documentation for further details.}

[asfimport]

Jonathan Keane / @jonkeane:
Issue resolved by pull request 10930
#10930