Fix example document paths in README quickstart
The two quickstart snippets in the README point at example documents that have moved.
example-docs/layout-parser-paper-fast.pdf(README line 121)example-docs/layout-parser-paper.pdf(README line 234)
Both files live under example-docs/pdf/ since 0eb461ac ("refactor: restructure PDF/Image example document organization", 2024-07-18). The top-level paths no longer exist, so copying either snippet as written raises FileNotFoundError. One of them is in the Docker quickstart, which is a first-run experience for new users.
This just adds the pdf/ segment to both. Verified the targets exist at example-docs/pdf/layout-parser-paper.pdf and example-docs/pdf/layout-parser-paper-fast.pdf on main.
Found by docproof, a documentation checker that verifies claims in docs against the repository and its git history. Hand-checked against the current tree before opening the PR.