Document Servant setup and combinators

[pcapriotti]

Added some documentation on Servant, which is hopefully useful for the upcoming Servantification effort tracked by https://wearezeta.atlassian.net/browse/BE-529.

Checklist

• The PR Title explains the impact of the change.
• The PR description provides context as to why the change should occur and what the code contributes to that effect. This could also be a link to a JIRA ticket or a Github issue, if there is one.
• changelog.d contains the following bits of information:
  • A file with the changelog entry in one or more suitable sub-sections. The sub-sections are marked by directories inside changelog.d.