Configure `--root-dir` or `--base` for lychee GitHub action

### Description Currently, the lychee GitHub Action fails when checking documentation that contains root-relative links. Example error: https://github.com/orhun/git-cliff/actions/runs/22559287619/job/65342428917 This happens because `lychee` is executed without a configured `--root-dir`, while the documentation uses Docusaurus-style root-relative routing.