Merge x-pack/docs into docs

[lcawl]

The X-Pack-specific documentation currently resides in https://github.com/elastic/kibana/tree/master/x-pack/docs, but in the long term (now that X-Pack is installed with Kibana), that separation is not required.

The Kibana User Guide documentation builds (and navigation tree) will be simpler if we no longer have to pull content from these separate directories.

The following items would need to be addressed:

• Ensuring that X-Pack-related code snippets can be tested successfully after the move. Not applicable, since there is no code snippet testing at this time.

• Can we stop generating OSS-only versions of the Kibana User Guide? They have not been provided on the web for a while now. If we can cease providing a method to generate OSS-only versions of the book, it avoids the need for complicated include flags. See [DOCS] Removes redundant index.asciidoc files #19192, [DOCS] Replaces OSS-only doc build command aliases docs#354, and [WIP] Updates Kibana Reference to use index.asciidoc docs#355

• Identifying whether it is appropriate to put the content from the "Setup X-Pack" section in the "Setup Kibana" section. See [DOCS] Moves X-Pack configuration pages in table of contents #19182

• Addressing broken links and creating redirects for all pages that are deleted or moved.

- [ ] Determining whether it is still necessary to highlight pages that contain X-Pack-specific content. Or perhaps only identify information pertaining to non-Basic X-Pack features? Currently, all X-Pack info has an icon next to the page or section title. For example:
. See elastic/docs#542

• Add "en" folder under kibana/docs (to contain all the existing english content there)? This currently exists in the kibana/x-pack/docs folder and was added to accommodate translation. --> Did not add this en folder; this decision can be revisited in the future if necessary.

See also elastic/elasticsearch#30665 and elastic/logstash#9596

[tylersmalley]

@lcawl is this issue resolved?

[lcawl]

@tylersmalley There's the one item in the list that's still under discussion. I'll close this issue when that item is resolved.