[docs-patrol] Document SKIP_PACKAGES in CI publish pipeline docs

Recent code changes in the repository have introduced documentation drift. The following change needs a corresponding documentation update.

## Changes Requiring Documentation Updates

### 1. `SKIP_PACKAGES` env var added to publish workflow but missing from CI docs

**Commit(s):**
- https://github.com/elastic/integrations/commit/62fb1c1152532b48f8ab957f6e6a5daf44cfc501

**What changed:**
- `.buildkite/pipeline.publish.yml` now sets `SKIP_PACKAGES` in the publish step (`.buildkite/pipeline.publish.yml` lines 41-43).
- `.buildkite/scripts/build_packages.sh` now reads `SKIP_PACKAGES` and skips matching package archives (`.buildkite/scripts/build_packages.sh` lines 9, 14-16, 73-76).

**Documentation impact:**
- `docs/ci_pipelines.md` currently documents publish env vars `DRY_RUN` and `SKIP_PUBLISHING` only (`docs/ci_pipelines.md` lines 104-106).
- `SKIP_PACKAGES` should be documented in the same section, including expected format (comma-separated zip filenames) and an example.

## Suggested Actions

- [ ] Update `docs/ci_pipelines.md` “Publish packages” environment variables list to include `SKIP_PACKAGES`.
- [ ] Document accepted value format and example usage for `SKIP_PACKAGES` (for example, `foo-1.0.0.zip,bar-2.0.0.zip`).

> [!NOTE]
>
> 🔒 Integrity filter blocked 2 items
>
> The following items were blocked because they don't meet the GitHub integrity level.
>
> - [#4564](https://github.com/elastic/integrations/issues/4564) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#784](https://github.com/elastic/integrations/issues/784) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
>
> To allow these resources, lower `min-integrity` in your GitHub frontmatter:
>
> ```yaml
> tools:
> github:
> min-integrity: approved # merged | approved | unapproved | none
> ```
>
>

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