Improve URLs in Validation chapter of the reference manual

[sbrannen]

Overview

In the Validation chapter of the reference manual, we have some good URLs and some not-so-good URLs.

✅ https://docs.spring.io/spring-framework/reference/7.0/core/validation/validator.html --> Validator

❌ https://docs.spring.io/spring-framework/reference/7.0/core/validation/beans-beans.html --> Data Binding

❌ https://docs.spring.io/spring-framework/reference/7.0/core/validation/beans-beans.html#beans-binding --> Data Binding

❌ https://docs.spring.io/spring-framework/reference/7.0/core/validation/beans-beans.html#beans-beans --> Data Binding / Property Binding

❌ https://docs.spring.io/spring-framework/reference/7.0/core/validation/conversion.html --> Resolving Codes to Error Messages

☑️ https://docs.spring.io/spring-framework/reference/7.0/core/validation/convert.html --> Type Conversion

Deliverables

1. validation/beans-beans.html is misleading. That should rather be called validation/data-binding.html.

2. validation/beans-beans.html#beans-beans is definitely inadequate: too many beans 🫘. That should rather be called validation/data-binding.html#data-binding-property-binding.

3. Although validation/convert.html might be better named validation/conversion.html, there is a connection to the core.convert package, so we can leave that one as-is.

4. However, having both validation/conversion.html and validation/convert.html does not make sense, especially with the former actual documenting error code resolution instead of type conversion. Thus, validation/conversion.html should rather be called validation/error-code-resolution.html.