[Task] Add empty router result diagnostic documentation

Related issue #10154 #9826

Status: Documents to be improved

Background: When a user requests the Dubbo interface, due to problems such as registry configuration problems, server status, and routing processing exceptions, no available server addresses are available for the request, and an error is reported.

Solution: Print out the processing process by improving logs such as log output registration center status, routing status, etc. to simplify the troubleshooting process. Currently, there is a lack of corresponding documentation.

Start by reading related issues #10154 and #9826 to understand the empty-router-result scenario and the proposed diagnostic information. Document how registry status, server status, and routing exceptions contribute to having no available server addresses, and make the troubleshooting process clear to users.

Written by the indexing model from the issue text.