Add Logstash output to functionbeat and refactor Logstash output docs

[dedemorton]

This PR:

• Breaks outputconfig.asciidoc into multiple files.
• Changes the libbeat-dir attribute to use docdir as base to make the paths more absolute (makes it easier to move files around).
• Moves shared-logstash-config.asciidoc content to a tagged region in outputs/output-logstash.asciidoc
• Adds a tagged region to outputconfig.asciidoc to make it easier for Beats that don't support all outputs to consume the content.
• Makes attributes used for conditional coding more specific.

[bmorelli25]

Good stuff here. A couple questions/recommendations.

Before I give this the green thumb, I need to make sure nothing explodes on the APM side. I'm going to check this out and run our make update-beats-docs command to be sure.

[bmorelli25]

Pending that include file change, these changes seem to work on the APM Server side!

If you're curious: elastic/apm-server#2744.

[bmorelli25]

One final APM Server request: Can you please comment out the setup-command-tag tagged region in command-reference.asciidoc so it doesn't render on the website 🤦‍♂. It's here:

beats/libbeat/docs/command-reference.asciidoc

Line 657 in dc99773

tag::setup-command-tag[]

And here:

beats/libbeat/docs/command-reference.asciidoc

Line 761 in dc99773

end::setup-command-tag[]

These should be changed to:

// tag::setup-command-tag[]

and

// end::setup-command-tag[]

This was my fault.

[bmorelli25]

Pending the changes above, LGTM!

[urso]

+100 on separate files per output. To me it looks like things got mostly copied around.

I wonder if this is also an opportunity to refine symbols used by our docs. Especially with settings like 'timeout', some hierarchy can be import to guarantee we always have the correct anchors.

For reference documentation (like the output), I'd like the symbol naming follow the actual setting names (makes it easier to search). For example: ref-output-elasticsearch-setting-timeout (I use 'setting', so to allow sub-section still be named ref-output-elasticsearch-<section>). Same for 'no' patterns: no-output-logstash. WDYT?

[dedemorton]

opportunity to refine symbols used by our docs

@urso That's a good idea, but I'd prefer to defer that level of refactoring because I want to have this PR merged by Monday in preparation for the release. I still have other stuff to do. :-/ I will rename the attributes as you suggest, but other renaming should wait until I have more time.

[dedemorton]

@urso I've added your request to the meta issue that I have open to track additional refactoring. We will need to be careful here because we don't want to inadvertently change anchor IDs that affect URLs (setting up redirects is possible, but kind of a pain). In fact, it might make more sense to do this for Beats agent rather than refactoring the existing content.

[dedemorton]

Doc changes have passed (no code changes) so I'm merging.