Add support for switching books to asciidoctor

[nik9000]

Adds support for switching individual books to Asciidoctor in conf.yaml.
It then cuts the glossary over to Asciidoctor. This is what the changed
docs look like:
https://github.com/elastic/built-docs-nik/commit/dfafa93c3c483bf74c2091f832c916f1ccc842ba

There are some line break changes which we can ignore. There is a change
to the HTML - in a <dd> AsciiDoc added a <p class="simpara"> tag.
Asciidoctor does not. The rendered result looks ever so slightly better
to me so I'm personally fine with it. The font size is 15px instead of
16px and the weight is 400 instead of normal.

[nik9000]

This is what AsciiDoc looks like:

This is what Asciidoctor looks like:

[nik9000]

I can merge this once it gets approval from @ddillinger (perl code) and @lcawl (different rendering). That would force a fairly important change: --all will start only being supported by the docker based build (build_docs, not build_docs.pl). If you have ruby installed --all will work with build_docs.pl today, but I suspect it won't for much longer. Docker will be required.

CI is perfectly fine with this as it is running docker 100% of the time now. But folks build this locally and they'll need to be comfortable with it. The thing that jumps out at me is that --all with docker basically requires an ssh-agent and --rely_on_ssh_auth.

[nik9000]

❤️ @ddillinger!

[nik9000]

Ooh! I just realized I should update doc_build_aliases.sh as part of this too.

[lcawl]

The asciidoctor version of the Glossary LGTM. Very exciting!

With respect to the need to use docker for --all, I suspect that it's not too common for folks to run the full build very often (particularly since it has historically taken so long). But I agree it's a good idea to double-check before we force their hand.

[lcawl]

One thought: Should/could this be a list of versions (akin to "branches") instead of a boolean? If so, it would give us more wiggle room if we can't convert some of the very old releases to asciidoctor builds.

[nik9000]

I'd be happy to do that when we figure out that we need it.