Automatic mechanism for associative links

Create an automatic system in Antora to assert a relationship between specified topics. Antora should then create associative links on each of those topic pages automatically.

The relationships may be defined in a relationships file (like nav.adoc) or within antora.yml: TBD. The important thing to note is that relationships are defined at the site or component level, not within the pages themselves.

Relationship assertions would look something like this (in yaml), where each topic might be a concept, a task, a reference, a tutorial or whatever:

relationships:

  - association: Transactions

    instances:

    - path/to/transaction_topic_1.adoc

    - path/to/transaction_topic_2.adoc

    - path/to/transaction_topic_3.adoc

  - association: CBO

    instances:

    - path/to/CBO_topic_1.adoc

    - path/to/CBO_topic_2.adoc

    - path/to/CBO_topic_3.adoc

Stretch goal: assert types of relationship, which create different styles of associative link; e.g. a sequence relationship, which creates first / previous / next / last links; and a collection relationship, which creates links to every other topic in that relationship.