Address Liquid errors in the generated sample-config file

[briandominick]

The sample config file generated during prebuild (build/docs/sample-config.yml) is missing some lines or displaying errors instead of others.

For instance, the conversions: block in the generated sample-config.yml is reported in the generated docs (as included in build/docs/sample-config.adoc, like so:

conversions: # Details about content origination, as well as markup sources and conversion.
  summ: issue # The source of the summary (Changelog) content.
#   head:  # The source of release-note headlines, when it is not the same as the summary.
#   note:  # The source of the release notes content.
#   note_custom_field:  # The name of the custom field to use for the release notes content.Liquid error: wrong number of arguments (given 3, expected 1..2)Liquid error: wrong number of arguments (given 3, expected 1..2)
  markup: markdown # The origin markup format for notes.
#   engine:  # The markup converter to use for the issues.

All of the observed cases of missing properties are caused by this Liquid error, but we should actually count the properties defined in specs/data/config-def.yml and make sure they're all represented in the sample config.