[feature] Custom Fields

We use CD to monitor sites that we copied documentation from (legally, we are partners of the content owners). But it would make life a lot easier if we were able to store the URL of where the content exists internally instead of having to go find it and include that URL in the notification content.

The issue names no file, test, or entry point. Start by tracing how monitored-site metadata is stored and how notification content is assembled, then determine the required custom-field and internal-URL behavior. Done means the URL can be stored and appears in the relevant notification without manual inclusion.

Written by the indexing model from the issue text.