[zh] add /zh/epub-contents.rst to fix sphinx build error
[zh] add /zh/pdf-contents.rst to fix sphinx build error
[zh] fix lang=en error in *contents.rst files
[zh] fix sphinx build warnings about nonexisting document in *contents.rst
[zh] try to fix sphinx warnings in build #1986
Merge pull request #1722 from blackpuppy/zh

[zh] fix sphinx build errors from last pull request #1721