Build LSR Plugin docs from this repo

[bmorelli25]

Why

As a part of our migration to Elastic Docs V3, we need to single-source the LSR docs from this repo. Currently, in the asciidoc system, the LSR docs are built from the logstash repo with this repo set up as a dependency. This PR moves the relevant documentation required to build the LSR docs from the logstash repo to this repo.

What this PR does

1. Copies the docs/include dir from elastic/logstash to this repo under docs/plugins/. The final dirpath is docs/plugins/include.

2. Copies the docs/static/core-plugins dir from elastic/logstash to this repo under docs/plugins. The final dirpath is docs/plugins/static/core-plugins/.

3. Finds and replaces:
   Find: ../../../../logstash/docs/include
   Replace: ../include

4. Finds and replaces:
   Find: ../../../logstash/docs/static/
   Replace:./static/

5. Finds and replaces:
   Find: ../../include
   Replace: ../../../include

Run with Asciidoc

$GIT_HOME/docs/build_docs --doc $GIT_HOME/logstash-docs/docs/plugins/index.asciidoc --chunk 1

Run with adoc-to-md

1. Check out this branch.
2. Update run.sh to:

/scripts/build_docs.pl --in_standard_docker --doc doc/logstash-docs/docs/plugins/index.asciidoc --chunk 1 --open --out out/md_docs

[karenzone]

Thanks for this work, Brandon. Is there any value to my generating updated docs for main so that we have the latest content in this PR?

Any updated plugin docs for main and 9.0 will need to have include_path: updated in the generated output PR (because that value is set in the logstash-plugins source doc.
For example: https://github.com/logstash-plugins/logstash-input-elasticsearch/blob/main/docs/index.asciidoc?plain=1#L11)

Another candidate for docs tooling later, and an easy manual fix for now.

[karenzone]

FYI: @robbavey @jsvd

[karenzone]

FYI @bmorelli25: Please don't backport to 9.0.
The [gemfile.lock.release](https://github.com/elastic/logstash/blob/9.0/Gemfile.jruby-3.1.lock.release) for each branch determines which plugin versions are appropriate for each release.

Does it make sense for us to generate and merge 9.0 plugin updates, and then migrate everything then?

[karenzone]

LGTM!
We're already in a place where we can't build stack-versioned plugin docs for main and 9.0 due to file changes/dependencies in logstash repo, so no worries about breaking capabilities.

IMPORTANT: DO NOT BACKPORT to 9.0 (related to gemlock file dependencies). Let's discuss.