Use a custom options renderer to add mixin headings instead of arg groups

[ajsutton]

PR Description

Switch back to using @Mixin to include options rather than everything using @ArgGroup and then use a custom options renderer to add in the headings.

Using arg groups affects how validation works which caused some weird situations, including potentially ignoring options.

Fixed Issue(s)

fixes #6146

Documentation

• I thought about documentation and added the doc-change-required label to this PR if updates are required.

Changelog

• I thought about adding a changelog entry, and added one if I deemed necessary.

[rolfyone]

LGTM