Logstash changelog test

[lcawl]

This is a temporary PR for testing purposes.
It plays with Logstash changelogs like this:

1. Install docs-builder
2. Create a changelog configuration file (based on https://github.com/elastic/docs-builder/blob/main/config/changelog.example.yml).
3. Create some changelogs, for example:

   docs-builder changelog add --prs 18377,18460,18265 --repo logstash --owner elastic \
   --products "logstash 9.3.0" --config ./docs/changelog.yml \
   --output ./docs/changelog --use-pr-number --strip-title-prefix

   That command fails with the following errors because two of the PRs were missing labels for us to derive the "type" from:

   The following errors and warnings were found in the documentation
   
   Error: Cannot derive type from PR 18377 labels (none). No matching label found in type mapping. Please provide --type or add pivot.types with labels in changelog.yml.
   
   Error: Cannot derive type from PR 18265 labels (backport-9.2). No matching label found in type mapping. Please provide --type or add pivot.types with labels in changelog.yml.

4. Create changelogs for the problematic PRs by explicitly specifying their "type":

   docs-builder changelog add --prs 18265 --repo logstash --owner elastic \
   --products "logstash 9.3.0" --config ./docs/changelog.yml \
   --output ./docs/changelog --use-pr-number --strip-title-prefix \
   --type bug-fix
   
   docs-builder changelog add --prs 18377 --repo logstash --owner elastic \
   --products "logstash 9.3.0" --config ./docs/changelog.yml \
   --output ./docs/changelog --use-pr-number --strip-title-prefix \
   --type feature

5. Optionally augment the description or any other fields in those changelogs (I edited the title and description in 18377.yaml but left the others unchanged from what was pulled from GitHub).
6. Bundle those changelogs for the 9.3.0 release:

   docs-builder changelog bundle --config ./docs/changelog.yml \
   --prs 18377,18460,18265 --repo logstash --owner elastic \
   --output ./docs/releases/logstash-9.3.0.yaml

7. Optionally create changelogs in the other plugin repos. For example, refer to Test changelogs logstash-plugins/logstash-output-elasticsearch#1239
8. Create bundles for the changelogs in those repos too. For example:

   docs-builder changelog bundle \
   --config ~/Documents/GitHub/logstash-output-elasticsearch/docs/changelog.yml \
   --directory ~/Documents/GitHub/logstash-output-elasticsearch/docs/changelog/  \
   --prs 1232,1228 --repo logstash-output-elasticsearch --owner logstash-plugins \
   --output ./docs/releases/logstash-output-elasticsearch-9.3.0.yaml

9. Add changelog directives to the existing release note pages. This is a one-time task and is a stop-gap for where we need to keep publishing in the current docs.
10. Generate a preview:

    docs-builder serve -p ~/path/to/logstash
    

Screenshot

Questions

• The original release notes had version info for each plugin. Is that still necessary in this new format (if so, will need to ponder how/if we can accommodate that).
• Refer to the questions in Test changelogs logstash-plugins/logstash-output-elasticsearch#1239