feat: [WIP] Support Document Index as a layout class

[cau-git]

Issue resolved by this Pull Request: Resolves #126

Checklist:

• [ ] Commit Message Formatting: Commit titles and messages follow guidelines in the conventional commits.
• [ ] Documentation has been updated, if necessary.
• [ ] Examples have been added, if necessary.
• [ ] Tests have been added, if necessary.

[mergify[bot]]

Merge Protections

Your pull request matches the following merge protections and will not be merged until they are valid.

🟢 Enforce conventional commit

Wonderful, this rule succeeded.

Make sure that we follow https://www.conventionalcommits.org/en/v1.0.0/

• [X] title ~= ^(fix|feat|docs|style|refactor|perf|test|build|ci|chore|revert)(?:\(.+\))?(!)?:

[cau-git]

Superseded by https://github.com/DS4SD/docling/pull/530