Improve handling of absolute URL path references in docs

There are links in docs to other projects using an absolute path (`https://www.elastic.co/docs/**`) instead of the cross-repository reference syntax (`[link](other-repo://path/to/doc)`), which bypasses processes like link validation. Consequently, these links are treated the same as any other link to external websites.

docs-builder should be able to identify links of interest and health check them, as well as provide feedback to the user if it identifies a link that should use the internal syntax.

RFC: https://docs.google.com/document/d/1JtC_Rbi8PnbAlnVw7GJIRWXI0rTsdN-weQzAQYyjiz4