* Add a guide to exporting events to Splunk
Closes#13158
Since there are multiple guides to exporting audit events now, this also
adds a new section of the docs for these guides.
Also fixes a tiny error in the Elastic Stack guide.
* Respond to PR feedback
* Fix linter issues
* Respond to PR feedback
- Use consistent naming
- Mention the `--ttl` flag for `tctl auth sign`
- Mention Machine ID
- Add a quick Troubleshooting section
- Fix `chmod` command
- Add systemd configs (also added this to the Elastic Stack Event
Handler guide. I didn't do this to the Fluentd guide because the
structure of the guide is different from the Splunk guide's).
