docs: explain how to add a new data source

We have a number of data sources and a fairly simple API for what's needed when you add one, but it currently isn't clearly documented.  This is a reminder for me to do that this week.

Here's a link to the explanation I wrote for the purl2cpe integration, which is probably most of the way to what we need in a document: https://github.com/intel/cve-bin-tool/pull/4164#issuecomment-2155170835
