doc-base file on Linux

[djcj]

At least for Linux a doc-base file could be interresting. CMake just has to add the path where the documentation is saved and save this file in /usr{/local}/share/doc-base/x265.

[Steve Borho]

our cmake scripts don't actually build the sphinx-based documentation, so my initial impression is that this detail is something that linux package maintainers would need to deal with.