[DOC] Document `unknown route` transactions

[gregkalapos]

From elastic/kibana#118695.

Summary: when HTTP requests hit 404 or do not map to an MVC controller, the agent generates transaction names with unknown route. We plan to help users to understand this in the UI and link to documentation about what these requests are.

Task:

• create a doc page explaining why the agent ends up with this transaction name.

[hegerchr]

Heads up @theletterf

[theletterf]

Would this be limited to the classic .NET APM agent?