[Docs] Update "Alert and action settings" docs to be generated from YAML source (#191787)

This PR:
- Updates the Kibana [Alert and action
settings](https://www.elastic.co/guide/en/kibana/current/alert-action-settings-kb.html)
page to be based off of a YAML source file
(`/docs/settings-gen/source/kibana-alert-action-settings.yml`) that is
manually converted to Asciidoc format
(`kibana-alert-action-settings.asciidoc`) by means of a Perl script
(`docs/settings-gen/parse-settings.pl`). A preview of the new, generated
page is
[here](https://kibana_bk_191787.docs-preview.app.elstc.co/guide/en/kibana/master/alert-action-settings-kb.html).
- Adds the `docs/settings-gen/parse-settings.pl` script which does the
YAML → Asciidoc conversion.

All new files are added to the `/docs/source-gen` folder. 

This is a trial run updating only one page of settings in the docs.
Later, in separate PRs, we plan to convert other pages. After all Kibana
settings pages have been converted, we would ask that the Perl script be
run automatically as part of the CI whenever the YAML files in
`/docs/source-gen` are added or updated.

**Notes:**
- The Docs team is happy to own and maintain the Perl script (sorry to
use Perl - it's the only scripting language that I know).
- In time we also plan to convert all of these files from Asciidoc to
Markdown.
- When we eventually/hopefully get the rest of the Kibana settings files
converted, we will announce the settings doc process to the Kibana team
by email and/or in the Kibana newsletter.

Big thanks to the amazing @lukeelmers and @KOTungseth for guiding this!

---
Why are we doing this? We aim to:
- Create a more consistent appearance for settings across all of the
docs.
- Make it easier for people to contribute, since all Asciidoc/Markdown
formatting is handled by a script.
- Make it more apparent which settings may be missing info, such as the
default values, available options, etc.
---

P.S. I haven't worked in the Kibana repo very much and would appreciate
any help navigating the CI checks.

Rel: https://github.com/elastic/docs-projects/issues/239

docs/setup/settings.asciidoc

@@ -653,7 +653,7 @@ Set this value to change the {kib} interface language.
 Valid locales are: `en`, `zh-CN`, `ja-JP`, `fr-FR`. *Default: `en`*
 
 include::{kibana-root}/docs/settings/ai-assistant-settings.asciidoc[]
-include::{kibana-root}/docs/settings/alert-action-settings.asciidoc[leveloffset=+1]
+include::{kibana-root}/docs/settings/alert-action-settings.asciidoc[]
 include::{kibana-root}/docs/settings/apm-settings.asciidoc[]
 include::{kibana-root}/docs/settings/banners-settings.asciidoc[]
 include::{kibana-root}/docs/settings/cases-settings.asciidoc[leveloffset=+1]