Reorganize rustdoc tests into their correct subdirectories for better
categorization:
- Move lint-related tests to rustdoc-ui/lints/
- Move intra-doc link tests to rustdoc-ui/intra-doc/
- Move deref-related tests to rustdoc-ui/deref/
- Move doc-cfg test to rustdoc/doc-cfg/
This improves test organization and makes it easier to find tests
related to specific rustdoc functionality.
