Update dev guide with ResourceNameCompleter

[maddieclayton]

Description

Checklist

• I have read the Submitting Changes section of CONTRIBUTING.md
• The title of the PR is clear and informative
• The appropriate change log has been updated
• The PR does not introduce breaking changes
• If applicable, the changes made in the PR have proper test coverage
• For public API changes to cmdlets:
  • the changes have gone through a cmdlet design review (Microsoft internal partners only)
  • the cmdlet markdown files were generated using the platyPS module

[markcowl]

I think you can use nameof(ResourcegroupName) here instead of "ResourceGroupName". This will make these easier to review, and also ensure that any name change is always reflected.

[markcowl]

Also, I think we should make this part of the requirements in the design guidelines. Also, I think we should write up a blog post on this new feature.