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

[mergify]

Contributes to https://github.com/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.

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

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[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.