WIP: Created an example `zathurarc` file.

I could not find any official zathurarc, so I followed along with the config.c recording as much as I thought relevant for most people. I write with a very particular style so that may not be liked. It is a bit disorganized; as such, I may rewrite it and split it up into about four files: zathurarc, normalrc, indexrc, and presentationrc Three of these would be called into zathurarc via the include <...> line—this would also then offer an example of how to do this.

My intent was to modify the meson file to tell it to copy doc/sample_zathurarc to /usr/share/doc/zathura/sample_zathurarc or /usr/local/share/doc/zathura/sample_zathurarc, but I did not know how to do that.

I believe I have submitted this request to the development branch properly.