[Developer] Add issue_comment workflow to fix lint/style/codegen

[asfimport]

Like https://github.com/r-lib/actions/tree/master/examples#render-readme.

• If changes to r/README.Rmd, render readme
• If changes to r/R, render docs
• If changes to r/src, lint.sh --fix

Reporter: Neal Richardson / @nealrichardson
Assignee: Neal Richardson / @nealrichardson

PRs and other links:

• GitHub Pull Request #6411
• GitHub Pull Request #6932

_{Note: This issue was originally created as ARROW-7801. Please see the migration documentation for further details.}

[asfimport]

Neal Richardson / @nealrichardson:
Issue resolved by pull request 6932
#6932