Add auto-generated changelogs

### Problem Description

The connector versioning is different from the main Elasticsearch stack, as we have releases that go down to the patch level.

Elasticsearch: `major.minor.patch` e.g. `v8.12.0`
Connectors: `major.minor.patch.build` e.g. `v8.12.0.1`

Patch-level releases have release notes that are covered by the Elasticsearch release cycle, but build-level release notes don't exist. This can cause issues when trying to determine what changed between build releases for connector clients.

These release notes should be auto-generated, possibly based on a release-note label. This is how it's done in the Elasticsearch repo. GitHub also has some support for auto-generated release notes we could look into.

```[tasklist]
### Tasks
- [ ] Investigate auto-generated release note possibilities
- [ ] (TODO: add steps here based on investigation)
- [ ] Update PR guidelines
```

Start by investigating GitHub's auto-generated release note support and the approach used in the Elasticsearch repository. Determine how build-level connector releases and a release-note label would fit the existing process, then update the PR guidelines with the resulting steps. Done means the approach is documented and build releases produce usable release notes.

Written by the indexing model from the issue text.