API update

[maddenp-cu]

Synopsis

• Expose uwtools.drivers.stager.FileStager via the API as uwtools.api.driver.FileStager to support apps creating external drivers using this abstract mixin.
• Expose uwtools.utils.processing.run_shell_cmd via the API as uwtools.api.utils.run_shell_cmd, and make the docstring more useful for end users.
• Update the documentation, including adding an API page for the new uwtools.api.utils.

Type

• Documentation
• Enhancement (adds new functionality)

Impact

• This is a non-breaking change (existing functionality continues to work as expected)

Checklist

• I have added myself and any co-authors to the PR's Assignees list.
• I have reviewed the documentation and have made any updates necessitated by this change.
• Where helpful, I have written comments in this PR's Files changed view to assist reviewers.

[christinaholtNOAA]

One very minor semantics comment that I'll let you make the call on, but otherwise this looks great!