[Doc]Add section for conceptual info

[karenzone]

Important info that customers need can be buried way down in the details. This PR adds a section for bubbling up and curating that information.

Replaces #11581

PREVIEW: http://logstash_11715.docs-preview.app.elstc.co/guide/en/logstash/master/processing.html

[karenzone]

@colinsurprenant FYI. From discussions in #11175. I believe that we have a good structure for moving forward. I'm ready to sync about new content for event ordering guaranties with multiple worker versus single worker.

This one replaces #11581

[colinsurprenant]

Great! We could start with something around the lines of:

• historically, logstash events ordering between their creation by an input plugin and output would be preserved when a single worker was used to execute a pipeline.
• Note that this behaviour was never officially documented but it was a feature that many users relied upon.
• That ordering was never guaranteed when multiple workers were used across threads because the time to finish processing events could vary between workers and the fastest to completion would be output before the slower ones.
• When the Java Execution engine was introduced, it broke the single worker ordering preservation. This was fixed with the introduction of the pipeline.ordered ...

[karenzone]

@colinsurprenant, your comments on event ordering were very helpful! I used some of them in this PR, and all of them in the release notes. Thanks for taking time to add them.

We also discussed that new section might be a good place to talk about Java pipeline initialization time. If somebody noticed a slowdown and googled "Java pipeline initialization time", what kind of guidance can we offer them? (Carried over from discussion in #logstash).

Note: I'm leaving this PR in DRAFT status until we flesh out a few more details. Your comments are always welcomed and appreciated.

[karenzone]

PENDING:

https://github.com/elastic/logstash/pull/11715/files#diff-fb62b697603cee2152850ac6aee1bec6R32

What should we advise people to do if they notice a slowdown?

[karenzone]

@colinsurprenant Please let me know what you think.
PREVIEW: http://logstash_11715.docs-preview.app.elstc.co/guide/en/logstash/master/processing.html

[colinsurprenant]

Thanks @karenzone! I left a few comments for what I thought was not clear to me, but I trust you to figure the best phrasing!

[karenzone]

@elasticmachine, run elasticsearch-ci/docs

[karenzone]

@colinsurprenant Will you please take a look to see if I captured your comments correctly? Thanks!

[colinsurprenant]

I think this might be slightly misleading to think that "When maintaining event order is important, use a single worker" and using a single worker is all you need.

Maybe something like:

"When maintaining event order is important, use a single worker and set 'pipeline.ordered => true'.

I think this will allow the this doc to live longer if we ever change the "auto" setting behaviour... WDYT?

[karenzone]

Nice clarification! Thank you for suggesting it.

[colinsurprenant]

Thanks @karenzone, looking good, left a minor comment!

[colinsurprenant]

LGTM

[elasticsearch-bot]

Karen Metts backported this into the following branches!

Branch	Commits
7.x	9aec969, 9597ab1, 335032d, 610abaa, 9bb792a
7.8	ee90e80, 77ad368, 7478c1c, 6c110a8, db7262e
7.7	ba2e6ec, 5e3310f, 5f8d318, d14143d, b7ddc30