Add custom exception assertions

Currently, the documentation shows how to extend an assertion in [2.6.2 Custom Assertions](https://assertj.github.io/doc/#assertj-core-custom-assertions) but there is no documentation for custom exception asserts, which seems to use a slightly different mechanism with some other class hierarchy.

It would be nice to include this in the guide and in the [examples repository](https://github.com/assertj/assertj-examples)

The goal for this section in the guide will be to present, given a custom exception, a way to assert on the specific fields in that class. For instance, given a custom exception:
```
package com.example.exception;

public class DetailException extends RuntimeException {

private String detail;

public DetailException(String message, String detail) {
super(message);
this.detail = detail;
}

public String getDetail() {
return detail;
}
}
```

and a business logic throwing this exception
```
package com.example.thrower;

import com.example.exception.DetailException;

public class DetailExceptionThrower {

public void throwException() {
throw new DetailException("message", "a detailed exception explanation");
}
}
```

write an extension to write a test like:
```
package com.example.thrower;

import org.junit.jupiter.api.Test;

import static org.assertj.extension.DetailExceptionAssertions.assertThatDetailException;

class DetailExceptionThrowerCustomAssertionTest {

private final DetailExceptionThrower underTest = new DetailExceptionThrower();

@Test
void test_customAssertion() {
assertThatDetailException()
.isThrownBy(underTest::throwException)
.withMessage("message")
.withDetail("a detailed exception explanation");
}
}
```

is it even possible to write an extension like this?

Start with the existing “2.6.2 Custom Assertions” section in the linked AssertJ guide and inspect the linked assertj-examples repository for its current assertion examples. Confirm how custom exception assertions fit the existing class hierarchy, then document an example covering the exception message and detail fields in both locations.

Written by the indexing model from the issue text.