Improve documentation on constructing the client

The documentation doesn't describe how the client is constructed using the database name, resource ARN and secret ARN. This seemed to have changed because [this](https://stackoverflow.com/questions/66145341/aurorards-serverless-with-data-api-library-in-java-does-not-work/70902387#70902387) example code doesn't work anymore because `RdsDataClient` no longer contains the described methods. I've searched a long time for examples but I'm unable to get the client to work.

Start by inspecting the repository's existing documentation and the current RdsDataClient API. Document how to construct the client with the database name, resource ARN, and secret ARN, and include a working example that reflects the methods currently available.

索引モデルが issue の本文から書いたものです。