Switches X-Pack Reference to Asciidoctor builds

[lcawl]

This PR updates the X-Pack Reference (https://www.elastic.co/guide/en/x-pack/index.html) such that it's built via Asciidoctor.

Related to #606

[nik9000]

This one is currently not building with:

INFO:build_docs:/tmp/docsbuild/target_repo/html/en/x-pack/6.2/index.xml:445: element partintro: validity error : Element partintro content does not follow the DTD, expecting ((title , subtitle? , titleabbrev?)? , (((calloutlist | glosslist | bibliolist | itemizedlist | orderedlist | segmentedlist | simplelist | variablelist | caution | important | note | tip | warning | literallayout | programlisting | programlistingco | screen | screenco | screenshot | synopsis | cmdsynopsis | funcsynopsis | classsynopsis | fieldsynopsis | constructorsynopsis | destructorsynopsis | methodsynopsis | formalpara | para | simpara | address | blockquote | graphic | graphicco | mediaobject | mediaobjectco | informalequation | informalexample | informalfigure | informaltable | equation | example | figure | table | msgset | procedure | sidebar | qandaset | task | anchor | bridgehead | remark | highlights | abstract | authorblurb | epigraph | indexterm | beginpage)+ , (sect1* | refentry* | simplesect* | section*)) | sect1+ | refentry+ | simplesect+ | section+)), got (titleabbrev )

Looks like some kind of bug. Not sure what though.

[lcawl]

@nik9000 Is that after fixing the issue with the path to the elasticsearch-extra folder per #505 (comment) ? Those are the errors I'm seeing with --lenient

[nik9000]

I was getting that error with --all rather than the --doc version. I've still got fixing --doc for this book on my list....

[lcawl]

I think it's the same error. I get a similar partintro error when I build the individual book without --lenient.

[lcawl]

The aforementioned error was caused by a <titleabbrev> after all, per https://github.com/elastic/x-pack-elasticsearch/pull/4479

[lcawl]

The documentation build errors seem to be solved by #721

[nik9000]

This seems to fail to build in the 5.6 branch.

[nik9000]

This one is now blocked on #824.

[nik9000]

I'm regenerating the diff for this one now.

[nik9000]

The differences seem to amount to:

1. Some different ids (bad but not a big deal in legacy docs)
2. A few ordered lists being reset when they shouldn't be (bad but not a huge deal in legacy docs)
3. A few ordered lists not being reset when they shouldn't be being reset but they were (good)
4. Table headings moving from the tbody into thead (good)
5. Lines with missing variables appearing when they were previously gone (good)
6. Some indentation changes around admonishments inside of callouts. (meh)

At this point switching to Asciidoctor is a net positive on the results so I think we should merge this. I don't think any of the problems are worth going back and fixing at this point. But that is just my opinion.

[nik9000]

@lcawl, I'm going to leave the merging this to you. If you agree with my reasoning then merge when ready1