DocsV3: Update plugin docgen tooling to work with new MD files and structure

Current stack-versioned plugin docs tooling generates plugin docs for inclusion in the Logstash Reference (LSR). I broke plugin doc generation (for `main` and `9.0`) when I [replaced asciidoc source files](https://github.com/elastic/logstash/pull/17289) in the `logstash` repo with the new MD source files. Tooling is looking for `logstash/docs/index.asciidoc`, but it's no longer there.

[Line `1574 Can't find --doc /home/runner/work/logstash-docs/logstash-docs/logstash/docs/index.asciidoc`](https://github.com/elastic/logstash-docs/actions/runs/13928870150/job/38980588712)

**Update:**
We have work-arounds that let us generate plugin docs PRs after:
- https://github.com/elastic/logstash-docs/pull/1829 + https://github.com/elastic/logstash-docs/commit/dec9b880823c00134e8a1dfdf0d072b806451f5b
- https://github.com/elastic/logstash-docs/pull/1839 +https://github.com/elastic/logstash-docs/pull/1842
The output will still fail docs-ci because of differences between ADOC and MD, but we have output to work with, cutting out several manual steps outlined in https://github.com/elastic/logstash-docs-md/issues/16..

#### Decisions
- Where should conversion happen? Source? Enrichment? End-of-pipeline?
- When to migrate source content?
- We'll likely need a phased approach (short-term, mid-term, long-term) to deal with complexity and volume. What does that look like?

#### Considerations
- Need ability to support both MD (for 9.0+) and ASCIIDOC (for 8.x and earlier).
- File location and `Include` path settings are different for 9.0+ and 8.x.
- Link format varies between ADOC and MD, and so far, has required manual cleanup

#### Tooling affected
- LSR plugin docgen:
- https://github.com/elastic/docs-tools/blob/main/plugindocs.rb
- [GitHub action in `logstash-docs`](https://github.com/elastic/logstash-docs/blob/main/.github/workflows/reference_docs.yml)
- Note that [tooling](https://github.com/elastic/docs-tools/blob/main/versioned_plugins.rb) for the Versioned Plugin Reference (VPR) is unaffected because it's self-contained. That is, it doesn't rely on any files in the `logstash` repo.

#### Related and relevant
- https://github.com/elastic/logstash-docs/pull/1817

#### Requirements
- [ ] Stack-versioned docs tooling should be modified to generate plugin docs for `main` `9.0`, and `9.x` branches
- [ ] Stack-versioned docs tooling should maintain existing (or equivalent) functionality to build plugin docs for 8.x series and earlier branches.