add docs on CI workflow inputs

[jameslamb]

Description

Contributes to rapidsai/shared-workflows#376

• adds descriptions for all inputs to workflows triggered by workflow_dispatch

Notes for Reviewers

Motivation

The input descriptions show up in the UI when you go to trigger these workflows. Like this:

I'm hoping that will make it easier for developers to manually trigger workflows. Inspired by being asked multiple times "what format is date supposed to be in?".

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.
• The cmake-format.json is up to date with these changes.
• I have added new files under rapids-cmake/
  • I have added include guards (include_guard(GLOBAL))
  • I have added the associated docs/ rst file and update the api.rst

[robertmaynard]

/merge