Fix anchor links being overwritten

[lewisbirks]

I noticed that the subsection MVC API - Execution Model was being used as the anchor link in the content for the Execution Model heading.

I think this is due to the tocItems method updating all links in the documents when the id changes rather than just those within it's element

jooby/docs/src/main/java/io/jooby/adoc/DocGenerator.java

Lines 411 to 420 in 5e345b5

	if (!id.equals(newId)) {
	h.attr("id", newId);
	doc.select("a")
	.forEach(
	a -> {
	if (a.attr("href").equals("#" + id) && a.attr("class").length() > 0) {
	a.attr("href", "#" + newId);
	}
	});
	}

I have updated this method to use h rather than doc and from what I can tell from a manual pass through, all links seem to work.

I've also moved the post processing functionality to make use of the inbuilt AsciiDoctorJ post processing extension, this should make the generation process slightly more efficient as it cuts down on the amount of file reads/writes being performed. As this is a separate change that is only tangentially related by being in the same area I can move it to it's own PR if wanted.

[jknack]

Thank you! Any enhancement here is always welcome.

[jknack]

All your changes are live, thank you.