Migrate slice docs into this project

[Kissaki]

Migrate slice docs into this project

[Kissaki]

Ice 3.6 still contains the slice2html executable. Ice 3.7 does not and tells you to use doxygen, which supports slice.

• https://doc.zeroc.com/ice/3.6/the-slice-language/generating-slice-documentation#id-.GeneratingSliceDocumentationv3.6-Usingslice2html
• https://doc.zeroc.com/ice/3.7/the-slice-language/generating-slice-documentation#id-.GeneratingSliceDocumentationv3.7-doxygenUsingDoxygenforSliceDocumentation