Add initial Sorbet linting; greatly improve YARD docs

[bensheldon]

• Using sord gem for generating RBI file from documentation
• Not yet packaging a good_job.rbi file

[bensheldon]

This PR is blocked by dynamically assigning a class parent:

good_job/lib/good_job/job.rb

Line 5 in 505d016

class Job < Object.const_get(GoodJob.active_record_parent_class)

I posted a question on StackOverflow following Sorbet's support instructions: https://stackoverflow.com/questions/67182040/how-to-hint-sorbet-with-const-get-and-dynamic-parent-class