Copied predefined paper size entries from scm/paper.scm as a seperate
appendix entry in order to better document predefined paper sizes.
Also revamped section on Paper Sizes including better @cindex entries,
reworded explanation and examples including how to create a custom
paper size without the need to edit scm/paper.scm
Added note in scm/paper.scm to say that the appendix entry was not
autogenerated so that any new entries will not be forgotten to be added
in the appendix - until someone 'scripts' it.