Add crosslinks to toc: in docset.yml

[theletterf]

This PR adds crosslinks as accepted item types in docset.yml toc directives.

For example:

toc:
  - file: index.md
  - title: External Documentation
    crosslink: docs-content://directory/file.md

[github-actions]

🔍 Preview links for changed docs

• docs/configure/content-set/navigation.md

[theletterf]

@Mpdreamz @bmorelli25 Wanted to give this one a try. Seems to work (check the Testing section, last two items).

If the code is ugly, please feel free to salvage whatever looks useful in this PR, or commit directly.

[theletterf]

@Mpdreamz Anything missing for this one? Minus the remaining comment.

[theletterf]

@Mpdreamz I've added a utility class to validate crosslinks, used for both the Markdown or in the nav.

With...

      - file: cross-links.md
        children:
          - title: "Getting Started Guide"
            crosslink: docs-content://get-started/introduction.md
          - title: "Title"
          - crosslink: https://www.google.com
          - title: "Item"
            crosslink: docs-content://get-started/item.md

The following warnings appear:

error::e.d.t.d.Log           :: Table of contents entries with only a 'title' are not allowed. Entry must specify content (file, crosslink, folder, or toc). Title: 'Title' (/Users/fabri/repos/docs-builder/docs/_docset.yml:135)

error::e.d.t.d.Log           :: Cross-link entries must have a 'title' specified. Cross-link: docs-content://get-started/item.md (/Users/fabri/repos/docs-builder/docs/_docset.yml:136)

Error: Cross-link URI 'https://www.google.com' cannot use standard web/protocol schemes (ftp, mailto, https, file, jdbc, tel, http). Use cross-repository schemes like 'docs-content://', 'kibana://', etc.
     ┌─[/Users/fabri/repos/docs-builder/docs/_docset.yml]
     │
 136 │           - crosslink: https://www.google.com
     ·             ─                                
     ·                                               
     │
     └─

[Mpdreamz]

This PR is finally failing like I was expecting it too :)

https://github.com/elastic/docs-builder/actions/runs/17245378751/job/48933555065?pr=1615#step:5:154

The sitemap builder does not yet know about the new CrosslinkReference type, will amend the PR.

[Mpdreamz]

LGTM but want @cotti to review as well since I did a bunch of changes here as well.

[cotti]

👍