[Idea] What's a gene identifier anyway? Add resources for people with this question

We always get these kinds of question. Why don't these gene identifiers agree? Might not be bad to find resources to send people to read more about this, since it is a consistent question. This might include adding links to any good articles we can find whenever we do a gene id conversion.

The issue does not name specific files; start by locating the gene ID conversion modules and reviewing how they currently present links or explanatory resources. Identify reliable material explaining why gene identifiers differ, then add appropriate references wherever gene ID conversion is discussed. Done means the relevant modules contain useful, consistent resources for this recurring question.

Written by the indexing model from the issue text.