Support for grouping tags with x-tagGroups

[CarltonHowell]

Is your feature request related to a problem? Please describe.

• Wanting to group tags in the generated Redoc HTML documentation
• See: https://github.com/Redocly/redoc/blob/master/docs/redoc-vendor-extensions.md#x-taggroups

Describe the solution you'd like

• Support for x-tagGroups to allow an extra level of tag grouping

Describe alternatives you've considered

• Modifying the spot generated openapi schema manually and run Redoc on it. This works but is clunky and would be nice to use the @airtasker/spot docs command instead.