Fix broken references to public doc_links

[gsoldevila]

The referenced file in the Kibana repo has been in a different location for a while.
A PR build recently failed, and I believe it might be caused by the outdated paths fixed with this PR.

🔨 Build docs PR
.buildkite/scripts/build_pr.sh
...
INFO:build_docs:Can't read /doc/product-repo/src/core/public/doc_links/doc_links_service.ts: No such file or directory at lib/ES/Repo.pm line 363.

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[lcawl]

Per slack conversation, I think the changes in build_docs.pl need to account for the location of the file in the old branches of the kibana repo. The current build error is:

Path 'packages/core/doc-links/core-doc-links-browser-internal/src/doc_links_service.ts' does not exist in '7.13'

[lcawl]

Thanks!