PDF build fails for ar, ko and ru

[rffontenelle]

Since PDF format was enabled in .readthedocs.yml, the pipeline of sphinx-ar, sphinx-ko and sphinx-run fails when generating the PDF file; HTML files build successfully, though.

See example of build failure:

• https://readthedocs.org/projects/sphinx-ar/builds/25539922/ (full raw build log)
• https://readthedocs.org/projects/sphinx-ko/builds/25539928/ (full raw build log)
• https://readthedocs.org/projects/sphinx-doc-ru/builds/25539925/ (full raw build log)

[rffontenelle]

For now, PDF builds are disabled (status badges went all green for HTML build only)