[zh] try to fix sphinx build warnings in build #1987
Merge branch 'master' into zh
[zh] correct meta data: change lang=en to lang=zh
[en] correct errors in original text
[zh] correct language: change en to zh in URLs
[zh] change PDF back to en PDF as zh PDF is not built currently
[en] improve the correction of the original text
Merge pull request #1730 from blackpuppy/zh

[zh] fix language errors and correct some errors in original text