Doc: Restructure authentication content in security section

[karenzone]

Restructure security content to group authentication options relative to each other.

Next steps: When content is nailed down, remove tags that force all security content onto one page so that individual topics appear in TOC (right nav).

Related: #13573

PREVIEW: https://logstash_13837.docs-preview.app.elstc.co/guide/en/logstash/master/ls-security.html

[karenzone]

@elasticmachine run elasticsearch-ci/docs

[karenzone]

@elasticmachine run elasticsearch-ci/docs

[karenzone]

Changing status to HOLD while I knock out other issues.