[docs-patrol] Update Metricbeat docs after removing `system.hostfs` config setting

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

## Changes Requiring Documentation Updates

### 1. `system.hostfs` config was removed, but docs still instruct setting it in config

**Commit(s):**
- https://github.com/elastic/beats/commit/6557b2fe0c928a16ed168ee0cc1edb03ba4f9e2e

**What changed:**
- `metricbeat/internal/sysinit/init.go:37-39` now unpacks only module-level `hostfs` via:
- `type HostFSConfig struct { HostFS string \`config:"hostfs"\` }`
- `metricbeat/internal/sysinit/init.go:90-100` (`findConfigValue`) only checks `hostfs` and no longer reads legacy `system.hostfs`.
- The same commit adds a breaking-change fragment explicitly stating removal:
- `changelog/fragments/1784889342-remove-system-hostfs.yaml:16` — `Deprecated system.hostfs setting is removed in favor of \`hostfs\` setting only.`

**Documentation impact:**
- `docs/reference/metricbeat/configuration-path.md:85-93` is now incorrect because it still says this can be set in config and includes example:
- `system.hostfs: /mount/rootfs`
- This doc should be updated to remove config-based `system.hostfs` guidance and example, and to direct users to module-level `hostfs` (while keeping CLI flag guidance if still supported).

## Suggested Actions

- [ ] Update `docs/reference/metricbeat/configuration-path.md` to remove `system.hostfs` config example and wording that says it can be set in config.
- [ ] Replace with module-level `hostfs` configuration guidance, consistent with the removal in `6557b2fe0c`.
- [ ] Ensure all `system.hostfs` references in docs are either removed or clearly marked as legacy/CLI-only where still valid.

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> - [x] expires on Aug 6, 2026, 3:00 PM UTC

Open docs/reference/metricbeat/configuration-path.md around lines 85-93 and compare its guidance with metricbeat/internal/sysinit/init.go and the removal note in changelog/fragments/1784889342-remove-system-hostfs.yaml. Remove the configuration-based system.hostfs example and wording, document module-level hostfs instead, and retain only supported CLI guidance. Done means the affected docs no longer present system.hostfs as a config setting.

Written by the indexing model from the issue text.