docs: fix dead doc links in i18n READMEs and the xlnet docstring (#48905)
- README_ro/tr/fa linked ./awesome-transformers.md relative to i18n/,
where no such file exists; the fr/de/ko/zh-hans/ja translations use
the absolute URL, and these three now match.
- The xlnet reorder cache docstring cross-referenced
PreTrainedModel.beam_search/beam_sample, which were removed; only
_beam_search exists.