[Only merge when 8.16 becomes current][8.x] Direct users to new API reference site (backport #5911)

[mergify]

Contributes to https:/elastic/security-docs-internal/issues/48.

Add callouts to the asciidoc API docs to direct users to the new API reference site, in preparation for retiring the asciidoc API docs.

NOTE: The api-kibana variable is defined in version-specific files. In 8.15.asciidoc and 8.x.asciidoc, the variable points to the v8 branch of the API reference, which currently doesn't include Security API docs. The v8 branch is derived from the "current" Kibana branch, which is currently 8.15. This likely means that we can only backport the callouts to 8.16 once 8.16 becomes the "current" docs version.

---

This is an automatic backport of pull request #5911 done by Mergify.

[github-actions]

A documentation preview will be available soon.

• 🔨 Buildkite builds
• 📚 HTML diff
• 📙 Preview page

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.}

[natasha-moore-elastic]

Per the note in the description, we should only merge this backport once 8.16 becomes current. Otherwise, as shown in this preview, we'd be directing users to a version of Kibana APIs that doesn't include Security APIs.