[Meta] Enhancements and fixes to Logstash doc tooling

Here's a list of fixes and enhancement requests that could save time, reduce risk of human error, and improve the quality of our documentation.

#### Strategic enhancements:
- [ ] #42 and related:
- [ ] https://github.com/elastic/ingest-dev/issues/4776
- [ ] #19
- [ ] #84
- [x] #90
- [ ] Possible workaround: #89
- [x] #84
- [ ] https://github.com/elastic/logstash/issues/13492
The new docs system is coming. We're breaking down product silos, and release notes/feature highlights/what's new will have a Elastic (not product) focus. The content will be easily searchable and filterable. The move to json output for release notes is an important milestone toward this effort.
- [ ] #109

#### Priority fixes/enhancements:
- [x] #51
- [x] #56 (was https://github.com/elastic/logstash/issues/12939 before transfer)
- [x] #57
- [x] #64
- [x] #66
- [x] [@mashhurs] #45
- Why? Plugins are not versioned, and all changes come from `main`. When we have mismatches with content in older versions, I have to remove regressions manually. The original design intent is that no new content gets picked up unless accompanied by a version bump. Re-implementing that behavior would solve this issue. Sample of manual workflow: https://github.com/elastic/logstash-docs/pull/1264
- PR: https://github.com/elastic/docs-tools/pull/72
- [x] [@mashhurs] #52
- Why? Doc tooling does not support aliasing. Keeping docs insync between input-beats and input-elastic_agent is a manual process for all branches. Samples of manual workflow: https://github.com/elastic/logstash-docs/pull/1212 and https://github.com/elastic/logstash-docs/pull/1255.
- PR: https://github.com/elastic/docs-tools/pull/68
- [ ] [@mashhurs] #46
- PR: https://github.com/elastic/docs-tools/pull/70
- [ ] #76
- [ ] [@mashhurs] #42
- [x] #80
- [x] #79
- [x] #67
- [x] #71
- [x] [@mashhurs] #69
- PR: https://github.com/elastic/logstash/pull/13971
- [x] [@mashhurs] #74
- PR: https://github.com/elastic/docs-tools/pull/75

#### Important fixes:
- [x] #53 The immediate need (aliased plugins) has been addressed with a specialized solution. Still, chances are good we might need this capability in the future.
- [x] #62
- [ ] #50
- [x] #49
- [ ] #43
- [ ] [#831](https://github.com/elastic/ingest-dev/issues/831): [ingest-dev][InternalDocs: Validate steps and update doc for manually generating plugin docs #831](https://github.com/elastic/ingest-dev/issues/831). Update: The problem was environmental. Need to document env requirements. Requirements and processes may change with new docs system, so on HOLD for now.

#### Reevaluate and triage:
- [ ] #34
- [ ] #21
- [ ] #18
- [ ] #7
- [ ] #1