doc: Enumerate configurations under each command

[abhinav]

For each command, enumerate configuration keys that affect it,
and link to the configuration reference.

This will be especially handy for config-only options
that don't have corresponding public flags.

[abhinav]

This change is part of the following stack:

• doc: Enumerate configurations under each command #329 ◀
  • Support setting GitHub URLs via configuration #330

_{Change managed by git-spice.}