Add Antora Atlas support to support partial site preview in the IDE

[ahus1]

Why the new feature should be added

When an author opens a single component repository, they won't be able to validate references in the IDE to other components, and xref titles in the preview referencing other components won't show the title of the target page.

The Antora Atlas extension stores metadata about a site, so afterwards a partial build can occur. This could be leveraged by the IDE.

How the new feature should work

• IDE retrieves the site URL from a playbook - alternative there could be a custom AsciiDoc attribute set for this
• IDE retrieves the site manifest can caches it locally
• When a component can't be resolved locally, it would use the site manifest to retrieve the information

Antora Atlas: https://gitlab.com/antora/antora-atlas-extension
Example: https://github.com/spring-projects/spring-security/blob/docs-build/antora-playbook.yml
Example Manifest: https://docs.spring.io/spring-security/reference/site-manifest.json