[9.0] [docs] Migrate docs from AsciiDoc to Markdown (#2582)

[marciw]

Backport

This will backport the following commits from main to 9.0:

• [docs] Migrate docs from AsciiDoc to Markdown (#2582)

Questions ?

Please refer to the Backport tool documentation

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[marciw]

@picandocodigo this is part of a docs-wide effort to make sure 9.0 branches are in sync with main. LMK if there's a better/preferred way to do this 🙏

[picandocodigo]

LGTM!

@marciw will there be a different place for Release notes in the new docs system?

[marciw]

LGTM!

thanks for the quick review!

@marciw will there be a different place for Release notes in the new docs system?

Release notes will be here https://github.com/elastic/elasticsearch-ruby/blob/main/docs/release-notes/index.md
... but things are evolving daily, so let me know if you need more info.