Switches Getting Started Guide to asciidoctor builds

[lcawl]

This PR updates the Getting Started Guide (https://www.elastic.co/guide/en/elastic-stack-get-started/master/index.html) such that it's built via Asciidoctor.

Related to #606

This PR must not be merged until #643 is complete.

[lcawl]

Verified that the builds against the 6.7 branch work now that #643 is merged.

[nik9000]

@debadair this one would change the anchor's id from _what_8217_s_next to _whats_next. Is that OK with you so long as no one is linking to it? I like the new text better and don't expect folks will link to the tag, but I don't fully understand any other implications of changing the anchor. I figure we change anchors from time to time when we're rewriting docs so my instinct is that it isn't too big a deal, but, like I said, I'm also keenly aware I don't know all of the implications.

[lcawl]

@nik9000 Actually it won't use either. I added an explicit anchor ID (whats_next) in elastic/stack-docs#228

[nik9000]

Ah!

[nik9000]

LGTM! The only difference is that Asciidoctor adds a nonbreaking space after the em dash.

[nik9000]

For reference, I ran my tests like so:

./build_docs --all --target git@github.com:elastic/built-docs-nik.git --push
<apply this branch>
./build_docs --all --target git@github.com:elastic/built-docs-nik.git --push --no_fetch
cd <a clone of built-docs-nik>
git pull
for file in $(find html/en/elastic-stack-get-started/ | grep \.html); do echo $file; docker run -it --rm --user 1000:1000 -v ~/Code/Elastic/docs:/docs_build:cached -v $(pwd):/work --security-opt seccomp=unconfined elastic/docs_build bash -c "cd /work && /docs_build/integtest/html_diff <(git show HEAD~1:$file) $file"; done

That spat out only the lines with the em dash.

[nik9000]

LGTM! The only difference is that Asciidoctor adds a nonbreaking space after the em dash.

s/nonbreaking space/zero width space/

[nik9000]

s/nonbreaking space/zero width space/

Still LGTM.