…is (elastic#192616)

## Summary

Closes elastic#188514

Adds OAS schemas for the `403 Forbidden` errors that public rule apis
can return if a license is invalid, `400 Bad Request` for unregistered
rule types, and `404 Not Found` for missing saved objects.

### Checklist

- [x] Any text added follows [EUI's writing
guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses
sentence case text and includes [i18n
support](https://github.com/elastic/kibana/blob/main/packages/kbn-i18n/README.md)
- [x]
[Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html)
was added for features that require explanation or tutorials

### Testing

1. Start ES
2. Add `server.oas.enabled: true` to `kibana.dev.yml`
3. Start Kibana `yarn start --no-base-path`
4. `curl -s -uelastic:changeme
http://localhost:5601/api/oas\?pathStartsWith\=/api/alerting/rule/ | jq`
(If you have `jq` installed, otherwise pipe to `pbcopy` and paste the
result into a JSON prettifier)
5. Search the output for the word `Forbidden` to ensure this schema has
been added to `create`, `update`, `enable`, `disable`, `mute`, `unmute`,
and `update_rule_api_key`

---------

Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>
Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>
(cherry picked from commit 18afcae)