[docs-patrol] Update CI pipeline docs for nested package paths

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

## Changes Requiring Documentation Updates

### 1. CI package-selection behavior now supports nested package directories, but docs still describe flat paths

**Commit(s):**
- https://github.com/elastic/integrations/commit/55c9cf43ab3903fb1d94b89daaa4c8213e50cd07
- Related rollout: https://github.com/elastic/integrations/commit/f8e677d6cb76452294b73803ca5997ed8c76d7b4

**What changed:**
- `.buildkite/scripts/common.sh` now resolves package directories via `mage ... listPackages` (`list_all_directories`, lines 883-885 in current file) and checks changed files against full `package_path` (`grep -E "^\$\{package_path}/"`, line 801).
- `.buildkite/scripts/trigger_integrations_in_parallel.sh` iterates `package_path` values from `list_all_directories` and passes full paths through test selection/execution (lines 53-57).
- `dev/citools/packages.go` `ListPackages` walks directories recursively and returns valid package paths, enabling nested package directories (lines 19-45).

**Documentation impact:**
- `docs/ci_pipelines.md` still states changed packages are `packages/*` (line 69), which implies flat one-level package directories.
- This should be updated to describe path-based/nested package selection (for example `packages/**` / full package paths), so contributors understand how CI decides impacted packages.

## Suggested Actions

- [ ] Update `docs/ci_pipelines.md` package-selection text to reflect nested package directory support (path-based matching instead of only `packages/*`).
- [ ] Add a concrete example with a nested package path to clarify CI behavior.

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