[8.12] Doc: Add structure and content for air-gapped architectures (backport #654)

[mergify]

This is an automatic backport of pull request #654 done by Mergify.

---

Mergify commands and options

More conditions and actions can be found in the documentation.

You can also trigger Mergify actions by commenting on this pull request:

• @Mergifyio refresh will re-evaluate the rules
• @Mergifyio rebase will rebase this PR on its base branch
• @Mergifyio update will merge the base branch into this PR
• @Mergifyio backport <destination> will backport this PR on <destination> branch

Additionally, on Mergify dashboard you can:

• look at your merge queues
• generate the Mergify configuration with the config editor.

Finally, you can contact us on https://mergify.com

[github-actions]

A documentation preview will be available soon.

Help us out by validating the Buildkite preview and reporting issues here.
Please also be sure to double check all images to ensure they are correct in the preview.

• 🔨 Buildkite builds
• 📚 HTML diff: Buildkite - Jenkins
• 📙 Preview: Buildkite - Jenkins
• 🧪 Buildkite vs Jenkins diff

Request a new doc build by commenting

• Rebuild this PR: run docs-build
• Rebuild this PR and all Elastic docs: run docs-build rebuild

_{run docs-build is much faster than run docs-build rebuild. A rebuild should only be needed in rare situations.}

_{If your PR continues to fail for an unknown reason, the doc build pipeline may be broken. Elastic employees can check the pipeline status here.}

[karenzone]

51373