Feature Suggestion: Add an example for updating a document

Hi there,

I was going through the spring-data-elasticsearch-examples and found them incredibly helpful for getting started. I noticed that the examples cover creating, searching, and deleting documents really well, which is great for the basic CRUD operations.

However, I couldn't find a clear example of how to update an existing document. For someone learning the ropes, seeing the standard pattern for "get a document, change something, and save it back" would be super useful and would make the examples feel more complete.

I have a couple of ideas on how this could be added cleanly:

• A new test in ElasticsearchOperationsTest.java that finds a conference, adds a new keyword to it, saves it, and then checks that the update worked.
• The same kind of test in ReactiveElasticsearchOperationsTest.java to show how it's done in the reactive world.

This seems like a straightforward way to demonstrate a really common use case without cluttering the main application code.

I'd be more than happy to put together a pull request for this if you think it's a good idea. Let me know what you think!