Update and improve documentation

[kriegaex]

Reading https://stackoverflow.com/q/77869970/1082681, it seems as if users are struggling with plugin configuration. The documentation says next to nothing about it and definitely does not resemble the (example projects](https://github.com/freefair/gradle-plugins/tree/main/examples/aspectj), such as this one:

gradle-plugins/examples/aspectj/aspect/build.gradle

Lines 1 to 22 in 982c8f5

	apply plugin: "io.freefair.aspectj"
	java {
	toolchain {
	languageVersion = JavaLanguageVersion.of(17)
	}
	targetCompatibility = "11"
	}
	dependencies {
	implementation "org.aspectj:aspectjrt:1.9.21"
	}
	compileAspectj {
	ajcOptions {
	outxml = true
	outxmlfile = "bar.xml"
	}
	}
	compileAspectj.ajcOptions.outxml = true
	compileAspectj.ajcOptions.outxmlfile = "bar.xml"

Instead, the manual seems to suggest using something like this:

task myAjcTask(type: io.freefair.gradle.plugins.aspectj.AspectjCompile) {
    aspectjClasspath.setFrom configurations.aspectj
    ajcOptions {
        inpath = files()
        aspectpath = files()
    }
}

Instead of having to answer user questions like the one on Stack Overflow, this one or #53, maybe it would be better to be a bit less terse in the documentation. It would be a one-time effort to improve it for the mutual benefit of yourself (less support work) and your users.